There are two use cases for which your phone number is required:
It allows us to quickly get in touch with you if we suspect fraudulent use of your account
It will be asked in order to set up the 2FA Validation.
It makes 3-D Secure protocol possible (i.e. the sending of a confirmation code for online purchases).
For these reasons, it is very important that you make sure that your contact details are always up-to-date in your user profile (learn how to update your information).