When you buy an official adventure from a company, the adventure comes with sections your meant to just read to the table. These pieces are generally descriptive writing sections that are their to paint the scene for you and your players in your imaginations. Often when doing custom work you designed yourself, I find a lot of GMs don't use those descriptions. The reason? They have it in their head, so the intention is to communicate it at the table to the players.
For years I did the same. I described things at the table as I felt appropriate there. Of late though I've also been dabbling with pre-writing some descriptions. I don't always read the description as written, but I keep close to it. I also find it helps. The page - or screen - gives something to focus on other than my players faces which helps me deal with that anxious/awkward feeling I sometimes get when giving descriptions as a GM. Also, with time to write it ahead of time, read it, review it, and then present it to the group I can be more sure I've caught all the important details.
I am curious about you and your table though. Do you pre-write descriptions to use in your custom content? If no, why not? If so, what got you to start or did you always do it?