|A woman drinking her Morning Coffee|
I expect there are a variety of reasons to drink decaf, but for me it's always one of two reasons that I turn to it.
For one, sometimes it's later in the day and I don't want the caffeine to keep me up that night. (Yes, I know I'm ignoring the fact that I've repeatedly shown myself capable of napping after a mug of coffee.) Sometimes I need it regardless of the time of day, simply because it's comfort food. This doesn't happen often, but I still like to keep decaf around for when such an occasion arises.
And then the other reason I turn to decaf is anxiety. When my anxiety is acting up and I'm on edge, with adrenaline ready to go at the slightest provocation, adding a stimulant into my system sort of goes in the Bad Idea category. If the anxiety is bad enough I won't be interested in coffee regardless of the form it takes, but if I do want a comforting mug of coffee then it'll have to be decaf.
So, yes, decaf does exist for a reason, and I'm sure there are also other reasons I don't know of to drink it. I still prefer regular coffee, but I'm also very glad that decaf exists.