Besides the points some have made (its more personal, it gives you options), I also find that a lot of people think long names come off as pretentious, and I can kind of see it.
And if it's inevitable that they will be given a nickname by others, I'd rather it be one that I like.
~☆March Song Recommendation☆~
NINETY ONE - "WHY'M":
https://tinyurl.com/252z6fvx