Those are all valid. For me, the two reasons I like to use one is:

• Better for social media with very limited characters
• You can choose the “slug” and make your source link into an easy to remember link that works anywhere (only on certain shorteners, I use one that lets you do this for free)
  Bonus reason I guess:
• When source link needs to be updated but you want to keep the same destination URL, you can do this via the shortener service and now you don’t have to update that link in the code or data of your websites that use it. (Some shorteners don’t support this, I use one that does)