There's no legitimate reason to use URL shorteners. It's only if you want to hide malware, tracking links, referral links, etc - none of which benefit the user.
Probably not that much, there's not much processing power involved nor bandwidth used (it's really just serving 301 redirects). A bare-metal dedicated server should handle quite a bit of traffic and remain cost effective (compared to their "cloud" equivalents).
There's no legitimate reason to use URL shorteners. It's only if you want to hide malware, tracking links, referral links, etc - none of which benefit the user.