Memo is a transaction identifier assigned by the exchange to each user. Exchanges only use one or a few addresses to receive ATOM tokens from multiple users, so they need to use the memo to identify the depositor. Therefore, when you send ATOM to an exchange, you have to fill in the memo.
However, the memo is not required when transferring ATOM from an exchange to imToken since imToken is a self-custodial wallet and each user has their own Cosmos (ATOM) wallet address.