As mentioned before, there can be many reasons.
Most of the times I have done it is because I am multi-tabling/multi-tasking, but sometimes I do use the time if I am in one big game and I need to think about an opponent's play. For example, if they raise me all-in, and I need to think about how many times they have done that before and had the nuts - sometimes that can take a long time to call or lay down.