On the planet Vulcan, there were people who wanted to develop Cyborgs. Those people were banned from Vulcan. They were the ancestors of the Romulans. The people who stayed on Vulcan were still passionate and war-like until Surak came along and taught them the True Way. Meanwhile the Romulans discovered what was wrong with developing Cyborgs but by then it was too late. The Proto-Borg escaped to the Delta Quadrant.
The Romulans then spent the next several centuries trying to develop a Quantum Singularity Drive. In hopes of being able to use it to eventually fight the Borg in the Delta Quadrant once they perfected it.
The Borg had intended to fight the Romulans when they attacked System J-25 in "The Neutral Zone" (TNG), but then shifted their focus to the Federation when it caught their eye in "Q Who?" (TNG). The Romulans then sat back and watched the Federation fight the Borg while gaining intelligence, so they could size up the Borg and how far they'd progressed from afar.
The Federation seemed to be able to handle the Borg but then they wanted to develop Synths. The Romulans remembered their own experiences and didn't want to deal with a second race like the Borg. So they manipulated the Synths to attack Mars so the Federation would ban all Synths.
Would that be the end of Romulus? Only if the Romulans didn't have another plan to save themselves. Most of them did survive. And the with a weakened Romulan government, that allows the Zhat Vash to use the Tal'Shiar to take more direct control. But the Tal'Shiar wouldn't be welcome with free arms, so they orchestrated for a new type of government to be formed and the Romulan Star Empire was replace with the Romulan Free State. Where leadership didn't change with coups. Where the government wouldn't radically alter from one administration to the next. Making the Romulan Free State more predictable than the Romulan Star Empire ever was.
They struck gold when they found the abandoned Borg Cube. Now they can do all the research they want on their creation.
And by infiltrating the Federation, they have control over their other greatest enemy.
So now: they have a more stable government, even though it was at the cost of Romulus, they're manipulating the Federation to their whims, and they've gained invaluable data on the Borg. Win, win, win.