Why DMs use screens

Look, I've been DMing for going on 20 years, and I like to think most of my players have had a good time. D&D isn't about adhiring strictly to the rules, it's about forging an entertaining adventure with your group. 90% of the time, I use the numbers I roll, but sometimes fudging those die can serve to make the game more enjoyable.

Is a boss battle not quite packing the oomph I intended it to? Maybe give the baddie a few good licks in to keep the players on their toes. Has someone been having a really rough time with an encounter and seems frustrated? Maybe just ignore that crit I just rolled. As long as you're not killing players, there's no harm in fudging the dice every once in a while.