Visa is a "ticket" into the country.
Once somebody enters the country, their visa doesn't matter as much. What matters is their status.
Students, for example, can stay in the country well beyond their visa expiration, as long as they have a valid status.
Once somebody files for adjustment of status, they can remain in the country until decision.
It's good to maintain other status, when possible. Let's say, I-485 gets denied. If this happens, technically an immigrant can be deported, since they're going to be out of status. But if they're still in valid status like F-1 student, they'll be allowed to remain on that status.