It's for security.
The latch bolt is angled and it can be opened with a credit card/knife/slim jim/screwdriver/etc.
But the small dead latch plunger can't be hacked as easily.
https://www.doorware.com/specials/help-center/faqs-door-locks.cfm
Q. What is a dead locking latch bolt?
A. A higher security latch bolt incorporating a plunger that is held in a retracted position when the door is closed, preventing it from being slipped open (by a credit card). Usually on keyed levers and knobs.