Spectre: GoG made the change a while back and a few other places have started doing the same over the last few years.
It seems the same with services wanting your mobile phone number.
Was there any deeper reason for this or they just playing follow the leader?
For any given example, I don't know what they were thinking, but from a network perspective, there are a few interesting reasons to sign in with email, or request a mobile phone number.
Two-factor authentication is generally held to be a security boon; it's easy for me to hack your email if you use one of the major providers, but it's a totally different task, requiring different technology and knowledge, for me to clone your SIM. Probably if I hack your email, I don't also have your phone number handy, as long as it has been stored safely >.>
Sales always likes to get information like that. Targetted advertisements have a higher hit rate (though still a vanishingly small one) than bulk sends, so advertising costs are reduced.
Follow the leader is persuasive, since that's usually how standards get formalized. As much as we'd like to think we're all precious snowflakes of independence, when in large groups, we work best with rules, guidelines, and expected behaviour - both as people, and as technology and information consumers.
Big data always wants more data. They'll always ask as much as they can, and then push to be able to ask more. It's almost always profit-driven, but almost never personal or intentionally malicious. There are a lot of really neat things that we can do now that you might take for granted, but would have been farfetched just five years ago, let alone ten. Almost all of the neat convenience features (like when you Google cookie shops and it gives you ones close to where you are) are a result of more information flowing back and forth.
The Information is the Spice.