The troper who has responded BUMP on a trope proposed in the Trope Launch Pad simply doesn't want the TLP to fall too far down the list and possibly be forgotten but doesn't have anything of substance to add.
If a TLP is BUMPed several times in succession, it's a signal. What it signals may vary. It could be an overanxious OP, worried because there have been no new replies in a while. It may be someone who thinks that it's ready to launch, but who doesn't want to launch it themselves. It may be a TLP that was having thoughtful discussion about a particular point, but the discussion stalled, and the BUMPer is trying to get it started again so it can be resolved.