Identity is an extremely abstract word. In imToken, the meaning maybe is a little different.
In the blockchain, the “digital identity” can be used to maintain the data that you stored in the blockchain, such as imToken's “address book” feature. You can add a friend's address to your "address book". This information will be stored in a distributed way(IPFS). Unless you own this identity, no one can use this data that makes your data inviolable.