Everything worth knowing, nothing that isn't.
Why shorten links in SMS
A single SMS is 160 GSM-7 characters, so a long link can cost you an extra billed segment. A short link preserves your character budget and keeps the message to one segment. See /answers/sms-character-limit-explained and /products/link-shortener.
Use a branded, trusted domain
Generic shorteners look like spam or smishing; a branded short domain tied to your sender ID keeps the link trustworthy and improves click-through. See /answers/what-is-smishing and /answers/sms-sender-id-best-practices.
Track clicks
Short links let you measure click-through per campaign — useful for attributing SMS marketing ROI when SMS itself has no open rate. See /answers/sms-marketing-roi.