You cancel attacks to make each wave last as long as possible and maximize interest. If you kill 28 creeps by the time the wave reaches the center of the maze, canceling attacks lets you stall for an extra 30 seconds and kill the remaining two at the last moment.The leaderboard is currently dominated by lifetower builds that reach 150k+ networth, and they all use this tactic from wave 1.
EDIT: OP, you can cancel life tower attacks by spamming S. It's pretty difficult to do, though. Maybe the top score guys use scripts or console commands, similar to the right-click one that was popular in Dota until the recent patch.
