Just trying to sum reasons for it’s popularity:
1)communication: ppl get to know when you have said something
2)Followers from anywhere: where do you otherwise find ppl who can follow you or you can follow. Someone from Antarctica too is following me, maybe.
3)medium / platform agnostic:
– I can tweet from web;
– from various desktop client-apps which have additional feature-integration like shorten url, add picture…
– from browser integrated apps / add-ons
– of course from mobile;
– i must have missed some more variations.
4)Information aggregator:I get so much of what’s happening around the people i’m following.