One of the most essential roles as a DM is to tailor your adventure in such a way that your players are having the most fun. Sometimes, for the sake of flavor, mood, or story you need a specific result to come up on the dice. In the physical analog this was achieved by rolling behind the DM screen and speaking the desired result aloud. Unfortunately on roll20 there is no way to make a roll appear fair to your players while simultaneously displaying the desired value. This leads to some really awkward player encounters where either something story-altering happens (which is a huge pain to rewrite) or you have to come up with some strange reason why things didn't actually happen the way the dice said they should. It kind of takes you out of the game a little bit when that happens. Seems like it would be fairly trivial to add something like this (only available to DMs): /roll 1d20+9 (17) Which looks like a normal roll of 1d20+9 but always results in a 17