The false bottoms are usually in the MLT, I think (and I'm sure someone will correct me if this is wrong) that brew kettles don't typically have false bottoms.
The false bottom in the MLT is to act as a filter for your grain bed. The grain is sitting on top of the false bottom, with (hopefully) clear wort underneath it - which is where the spigot will be. It prevents large grain particles from getting into the brew kettle. Alternately you may also see a stainless steel braid which will also act as a filter.