Well, the Borg's goal is assimilation of societies with advanced enough technology that they can add to the Collective's "perfection." Therefore, 24th century Earth is a lot more appealing than Earth of any century previous.
The question ultimately becomes why did the Borg bother with time travel in First Contact at all, especially to such a low-tech period of Earth. Not a lot about First Contact makes sense, and up until May 2009 it had the most amount of plot holes for a Trek movie.
The big question is why the Borg were building the interplexing beacon. it wasn't to call for reinforcements. I mean, send a single drone down to Earth. Each day, it assimilates one person. The next day there are two drones. Each assimilates one person, and on the third day there are four drones. Keep going and the entire population can (in theory) be assimilated within a month.
Also take into account that the people back then have very little in the way of defenses against the Borg.
And also take into account that you'd be starting with at least a dozen drones if the Borg beamed down to earth when the sphere was destroyed instead of to Enterprise.
So what did the Borg need reinforcements for? they didn't! And even if they did, they could have kept assimilating the Enterprise crew until they got everyone. That would give them more than enough, as well as lots of technology that far outstripped anything found on earth.
And even if they did, were the Borg really just going to wait around for however long it takes for a Cube to arrive from the Delta Quadrant?
So no, the Borg were not calling for reinforcements.
In fact, i don't think that they wanted to stop first contact at all.
I mean, think about it. What did the borg do in their efforts to stop first contact? The were shooting the silo at the beginning, and the Queen got Data to shoot at the Phoenix at the end, but honestly, those efforts seem half-hearted. The Borg Sphere should have been able to obliterate Montana itself with a few shots, but they didn't. Heck, even kirk's Enterprise had the ability to destroy all the life on a planet, and we'vce seen it in NextGen as well (The Chase). The Borg could do it if they wanted to, and they didn't. And the Queen could have got one of the drones to fire the torpedoes at the Phoenix, but she chose to let data do it, even though she couldn't trust him.
She just wasn't trying very hard.
And in the middle of the Movie, she puts all her effort into building a big radio!
And the final proof that this is correct...
Once the interplexing beacon is destroyed, the Queen tells Data that there's been "a change of plans". And it is only then that the Borg start their attempts to prevent First contact again! Why? because the Queen thought that her attempts to contact the 21st century Borg had failed, so she decided to assimilate Earth so there would still be a Borg presence in the Alpha quadrant. And who knows, once earth was assimilated, the Borg would probably be able to build an interplexing beacon anyway. But with Enterprise's nav deflector out of the picture, the Borg on Enterprise had no way of building one. It would have to wait. And so the queen changed her plans.
So what were the Borg doing with that interplexing beacon?
It wasn't to call for reinforcements. And it couldn't have done anything to interfere with Cochrane's warp flight.
But it would have been ideal to send the Borg of the past information about the future. Specifically, information about two events.
The first was the events of Q-Who?, in which a starship apparently used a form of transport that was much more effective than even the Borg’s transwarp drive (even though it was due to Q moving the Enterprise back to Federation space). The second was in the first main Borg attack on the Federation, when a human named Jean-Luc Picard formulated a defence against the Borg of such particular cunning that the Collective suffered a major defeat.
These two facts would have been included in the message that the Borg sent to the Delta Quadrant, thus informing the Borg of the 21nd century that Earth had apparently developed into something much more interesting, and worthy of assimilation. It was this message that ensured that the Borg would be curious enough about Humans and the Earth to send the Cubes that were seen in Q-Who?, and The Best of Both Worlds. Even though the Borg knew that it would cost them the ships they sent, they would have realised that such valuable knowledge about the future and a potential target would have been worth it. Thus, the primary mission of the Borg sent back in time during the film Star Trek: First Contact was not to assimilate Earth in the past, rather it was to ensure that drones with knowledge of the future were in a position to send that knowledge to the Collective in the past! It was a pre-destination paradox that was in the Borg’s best interest to ensure occurred. When viewed in this manner, it becomes apparent that the Borg’s primary mission in Star Trek: First Contact was actually completely successful (simply delayed), and that is the reason why the mission was launched, despite the fact that destruction of all the resources involved was assured.
Finally, as to the question of why the Borg travelled to Earth before travelling back in time? The answer is simple. If the Borg went back in time while they were in the Delta Quadrant, they'd probably find that the transwarp network wouldn't reach as far as earth.
So why go to Earth at all?
Because the Borg would then be in a perfect opportiunity to assimilate earth. true, it was a secondary mission objective, but if the Borg had succeeded, then they called have called the Borg in the 21st century Delta quadrant to earth, and they would have found a planet already assimilated AND with Borg technology 300 years more advanced.
(BTW, this also explains how Seven knew the Borg were present at first Contact, as Capt M said.)