1. To make your philosophy on life clear to the world.
2. If you're getting sick of repeating yourself in answer to the same old questions: 'what's that on your face?', 'what does your tattoo mean?', 'Are you pregnant?', 'Where have you been for the last year?', 'What do you do for a living' - Get it out the way with without saying a word.
3. To support a cause. Any cause, even if you're the cause.
4. For the sheer hell of it.
5. For your holiday or weekend away.
6. If you're on the pull and want to cut to the chase. Nothing wrong with a bit of self-marketing.
7. If you're going to a gig and don't want the same t-shirt as everyone else, you are unique afterall!
8. You could have a t-shirt for every day of the week: 'Monday', 'Tuesday'.... So you can minimilise your wardrobe. Less clutter externally means less clutter internally :p
9. For your kids, so you can easily spot them (or lose them) in a crowd.
10. For your Boyfriend/Girlfriend - either as a romantic gesture or to make sure everyone knows they're taken.
11. If you're doing something for charity and want everyone to know you're an excellent person.
12. If you want to be left alone: 'Leave me alone'.
13. A custom t-shirt can say everything you want to say in a oner.
14. If you've left it too late to get your best friend a Birthday prezzie, getting them a customised t-shirt is going to look so much better than a box of malteesers and a lucky dip.
That's my mind dried up for now, if anyone has any better suggestions I'd like to hear them! :)