Email account providers request a mobile phone number - some services from Google have demanded one or you can't register the account - so that the password recovery function of their services is more simple and reliable.
The standard password recovery function for most email accounts is for you to provide another email account, and they will send your password to that account if you have forgotten it. The problem with this system is that people often forget the password for their backup email account, and, therefore, the password recovery function can't proceed.
The advantage with a mobile phone recovery function is that people tend to stay with the same mobile phone number for longer than they do with a backup email account; which means the password recovery function has a greater statistical chance of success.
Another reason why many online services, and that includes email, require users to enter a mobile phone number to create the account: is to verify the identity of the user and ensure that the account is not going to be used for negative reasons; like spamming or harassment.