In what scenarios would the Grapplers be more useful than Tractor Beams?

[Tosk]

Tractor beams only extend about a ship's length

That's what we typically see, but that is not their maximum range. We've seen more than one example that shows tractor beams having a much longer reach than that such as the Enterprise towing the barge in Final Mission, or DS9 capturing Tandro's ship in Dax.

 

[Sisko_is_my_captain]

The Defiant lassoed a Klingon ship that was firing on the Cardassians in WotW.

 

[FederationHistorian]

Using it as a way to board another ship during high warp, like Trip did during "Divergence ". It would be especially beneficial if the transporters was down on both vessels, and docking was impossible.

 

[Timo]

Heck, even shooting a transporter cable through the enemy shields might be useful and doable! If the thing can be made to physically penetrate, the transporter signal then goes in and allows for boarding of a shielded ship. Not too many tactical situations where you can fly in formation with a shielded opponent, but there are a few in Trek history.

Timo Saloniemi