[Dakota]

Originally Posted by Skahtboi

From the 2009 CCA Softball Umpires Manual:

"Once the plate umpire is satisfied that the lineup contains the necessary information, she should hand it back to the coach for a final look."

From the NFHS Softball Umpires Manual:

"Make certain you check each lineup card including first, last name, defensive positioning and number of each participant and hand it back to the coach for a last inspection."

In other words, you do it because it is what you are instructed to do. Once the lineup is handed back to you, it becomes official, and any problems that have been hitherto missed by the coach can now be penalized.

From the 2009 ASA Umpire Manual:

D. Plate umpire receives line-up cards from each coach.
1. Inspects for proper number of players in the starting line-up
2. Inspects for proper information, by rule
3. Returns line-up card to respective coach for final check
4. Accepts as "official" when cards are returned to plate umpire