Your phone number is requested for a few reasons:

1)  It allows you to login by using your phone number (easy to remember) or your username

2) If you forget your password, you can request to have it sent to you via a text message

3) If you want to receive notifications of newly shared or uploaded videos or photos, you can enable an option to be alerted via text messages, sent to the mobile number that you provide.

Note: Thwapr is exploring mobile phone numbers as an optional part of registration, please stay tuned for updates on this topic.