It's also so you can get the jump on risks as early as possible. Consider two projects, both roughly a week long. Project A is broken down into day-long tasks, and project B isn't. On day two, the guy doing project A reports he wasn't able to get milestone 1 completely done yesterday. Others can now step in and help if needed, after only a day. The guy doing project B, who reports "analytics yesterday, analytics today" needs to subjectively assess whether or not he's behind, and depending on how experienced he is at this, nobody will know until the sprint is blown that he was unrecoverably behind.
