For contrast, so you can see. The typical opening break is off one of the corners. The corner balls are partially hidden by the ball in front. The break is not the desired shot. The winner of the lag usually makes the other player break.
When I first started playing straight pool a veteran player taught me to rack with the 1 and 5 on the corners and the 3 on the spot so the breaker can see them better.
And I usually put the 2,4, or 6 next to them, as Frank said, for contrast.
A note...
I refereed during many tourneys back in the 60's and 70's...I was taught by the "old timers" that the two opponents ..."lagged" using the 5 and 1...not to give an edge to either and those balls would then be placed into the rack on the empty corners that were saved for these balls ..and they would be easy to see for the break shot...normally the higher numbered distracting colors were placed at the top of the rack...