Beyond the nefarious, you could:

1. Have a cheap secondary number for SMS verification. You can send free SMS from the same number if you link it to a service like smspup.
2. Legitimately provide it on websites asking for your contact details.
3. Use it for 2FA on various websites, to save your US-prepaid credit.
4. You could setup SMS notifications when your torrent finished downloading (guide here)
5. Use it in a gadget where you send SMS commands.
6. Have an active Australian cell number before you travel down-under.
7. Trick your friends into thinking you moved to Melbourne, Australia.

I'm sure I could think of other reasons, if you want more