Brian Dunne speculates on why Israel is ordering 100 bunker buster bombs. Common speculation is that they're for Iran's nuclear program. He thinks that this is not correct but doesn't come up with an alternate use for them.
Any reasonable speculation on Israel's US defense purchases really needs to have a use for new weaponry. The US is restricting Israeli technology transfer because we've found Israel selling on tech in violation of agreed upon restrictions a few times too often. So not only is it interesting why is Israel buying such weapons but why are we selling them the Israel?
I think that those bombs are for deep tunnels into Gaza used by Hamas and others to run arms from Sinai. Instead of having to invade Gaza, find the surface point, battle your way through the tunnel, and collapse it from inside with explosives, you simply launch a plane, bomb it, collapse it without any warning and no surface casualties.
Such strikes would be a politically superior solution both raising the risk for palestinian use of such tunnels and also lowering Israeli risk. Depending on the cost of the munitions, this solution might actually be cheaper in strict dollar terms as well.