First off, floor and ceiling are not editable, so they will be default lineups unless you use some other stuff along with it (locks, exposure, lots of exclusions etc....)

The floor and ceiling of players are inversely related. This is because it indicates the volatility of the players. You obviously would not want to optimize an entire lineup based on floor/ceiling, because you're making the assumption that everyone in the league will hit their ceiling/floor, which is absurd. So let's say you have most of your players figured out 75% or something like that. You may ask yourself "what is the safest way to fill up the remainder of my players?" You may want to use the floor calculation to fill the lineups with players that are consistent or have high floors. Similarly, if you are playing in a tournament, and you've picked out your "core" players that you think will be solid you may ask yourself "Who are some players that I can fill in my lineups that have a decent shot of exceeding expectations?" Then you could fill in your remaining slots with a ceiling calculation.