Popular lore has it that the armies clashed at Gettysburg because of its vital road system. Gettysburg was indeed attractive from a military standpoint, with its 11 roads radiating from the town like spokes on a wheel.
Moving a huge army like the Army of the Potomac on a single road might require a column 75 miles long. If the head of the column encountered trouble, it could literally take days for the army’s tail to make it to the scene. But having multiple roads allowed a commander to split his army into shorter columns which could then march on separate roads concurrently – thus allowing much faster movement, plus it made for a much more powerful striking force when the disparate columns suddenly converged at a single point.
All that said, it’s still an open question as to whether the opposing armies clashed at Gettysburg because of the town’s strategic road system, or whether Gettysburg just happened to be where they bumped into each other. Neither army even knew where the other was until some of Buford’s encountered Heth’s men west of Gettysburg on July 30th. Also, one of Lee’s divisions, commanded by Jubal Early, had marched through the town a couple of days earlier and left. Had the town been on Lee’s mind as being important, presumably the Confederates wouldn’t have abandoned it.
It’s more likely that it just happened to be Gettysburg where the two armies stumbled into each other.
And although the town’s roads were important, what really turned out to the critical terrain was the ridgeline south of the town, Cemetery Ridge, which was an ideal defensive position, and which the Union Army wisely hung onto for dear life.