We originally permitted disposable email addresses to be used. However, we found that users would often lose access to these email accounts and then lose access to their games. This was a support problem for us and was a bad experience for users.
Worse still, we had users who would forget they’d signed in with a disposable email address and then they’d make a purchase of a Steam key, which we would dutifully send to them by email. The user would have no access to a proper email inbox and would then struggle to get the game.
For those reasons we do need everyone to have a valid email address to use the service. We don’t sell these to anyone and they are used solely for your use of the service and for whatever level of marketing information you chose to opt-in to receive. We work hard to ensure we comply with the GDPR/EU Data Protection Directive for all users, whether within the EU or not.