I can think of two instances where this comes into play. One is if the kick-off falls behind the receiving goal line, and the runner chooses to "down" the ball in the endzone rather than run it out. This allows the receiving team to start at 20 yard line "for sure" instead of taking their chances with a running play.
The second is if the offensive team's ball carrier is tackled in his own end zone for a "safety," this team has to make a free kick from its 20 yard line (instead of from the 40 yard line).
How and why was the 20 yard line chosen for these purposes? And why was the "20" yard line changed to the 25 yard line for both touchbacks and safeties in college football in 2012?