(Reverse) break-up fees are a tool to ensure commitment of both sides to a deal. The main target in the Twitter deal was probably the financing, not the regulatory case, making it a bit less attractive to fail the financing for the deal somehow to get out of it (although there was some debate how realistic that option was anyways). And in the other direction, Twitter was on the hook for a billion for various things it could've done to hurt the deal, e.g. the board recommending shareholders vote against it, or for a competing offer.