Lock conflict

[席冲(宜穆)]

Hello all,
This PostgreSQL version is 11.9.
In LockAcquireExtended(), why if lock requested conflicts with locks requested 
by waiters, must join
wait queue. Why does the lock still check for conflict with the lock requested, 
rather than check for directly with conflict with the already-held lock?
I think lock requested only check for conflict with already-held lock, if there 
is no conflict, the lock should be granted.
Best regards