In Star Trek First Contact, why did the Borg go to Earth, then back in time, couldn't they have time travelled first?

If the Borg possess the ability to time travel as we have seen, why would they travel in space to their target (i.e. Earth) then back in time, could not they do it from the Delta quadrant? That is move back in time in the delta quadrant and then travel to any planet to assimilate. No combat, battles or anything. Or would that create paradoxes?

Answer

The novel Engines of Destiny provides a possible reason. From Memory Beta on Species 1429:

From 1429, she gained the time travel technology that she utilized aboard the Borg sphere that traveled back in time; however, the device malfunctioned - where she had set it to take her only a few days in the past, it returned her several centuries. After this, the Queen decided that she would only utilize it in an extreme emergency, as to prevent further malfunctions.

The movie First Contact opens with a battle against a Borg Cube, which is destroyed. The Sphere detaches and goes back in time as a last-ditch effort as the Cube is destroyed. This (non-canon) quote provides a possible reason - although it's not clear whether or not the sphere that Memory Beta refers to is the one from First Contact, or another one the Queen was testing the technology on.

A couple days may well have been the intent of the Sphere from First Contact; it could have contacted the Cube before it was destroyed and altered the outcome, without major changes to the timeline, which wouldn't necessarily have been a desired outcome for the Borg considering the numerous conflicts they've already had.

_{(Aside, preventing humans from reaching space before the Federation was created would have been quite a bad mistake on the Borg's part, since their conflict with Species 8472 would likely have already started by that time (Scorpion takes place on Stardate 50984.3, and First Contact on Stardate 50893.5). If there was overlap between Voyager's assistance and First Contact, their primary goal would've been to destroy the Federation after Voyager arrived in the Delta Quadrant. This also would explain why the Borg in Regeneration didn't stay on Earth and try to assimilate the planet...)}