Inside the tank, at the bottom, is a bracket with a hole that the "nub" on the bottom of the strainer fits into; the strainer attaches to the bottom of the pump, and is the inlet to the pump.
This hole has a large grommet that supports the fuel pump assembly, and keeps it from moving arround. It's possible to have the grommet pull out of the hole when you remove the pump, and possibly fall into the tank.
One problem with replacing the fuel pump without dropping the tank, is that it's difficult to get the fuel pump assembly back into this grommet. To me, it's worth the extra work to drop the tank, and while it's out, steam clean the inside.