Providing a phone number during registration is optional, and can serve many purposes.
1. It helps with Smart Matching of your contacts.
2. It serves as a unique identifier, just like username and email are used in other services and social networks.
3. It allows you to have a "ME card" (Sync.ME Profile) - Sync.ME users who have your phone number in their device can sync with your ME Card and receive your updated contact details as you edit them.