In what situation would someone remove their Guard?

VGO's answer is the correct idea, but it relates more to shields, and the question specifically asks about guards.

Since guards are only 15 minutes for the vast majority of clashers, there is no real benefit to dismissing the guard for these people. In 15 minutes at TH9 (these numbers will be even less at lower THs), your collectors will accrue:

Gold/Elixir: 6*(3500*0.25) = 6*875 = 5250

Dark Elixir: 2*(100*0.25) = 2*25 = 50

Where 50% of Gold/Elixir is lootable, that's only 2625 gold/elixir

Where 75% of Dark Elixir is lootable, that's only 38 dark elixir

These numbers are quite low and shouldn't matter much; however over time I suppose they could add up. Although, sometimes having a small amount in the collectors can trick attackers into thinking there is more in there than there really is and can help your base defend, so NOT removing your guard could(?) sometimes be better.

In any event, at higher leagues where guards last longer, it becomes more reasonable to make sure to dismiss your guard before you go to bed. They last hours at high leagues like Titan, during which time your collectors actually could accrue a more meaningful amount of money, so it makes more sense to dismiss the guard.