It's unnecessary, I've always found it relatively easy to identify if someone is a biological male or female from either looking at them or checking their name, or both.
So when I see an email signed off 'John Smith - he/him' I can't help but think well yeah... obviously.
I think people like that are just trying to normalise the practice to not make others feel strange for doing so.
Apart from the gender point that others have made: while it may be obvious for names from your own culture (probably western names given your username), names from other parts of the globe may not be so obvious (I have no idea which Chinese teams are male or female, for example); you don't always see each other with today's virtual environments; and it's not always clear from appearances either (which, again, is further complicated by gender).
It's easy for me: my name is instantly recognisable as male here in Canada, I look male (beard) and I identify as a man. But I anyway put those pronouns in my e-mail signature to normalize the practice (as
@horsechoker said), so that it's easier for people who think they need to give their pronouns to do so without immediately feeling like they're stigmatized themselves.
See it as a kind of community service.
That's the point though - if I call pen1s owning Sam Smith a man that is not misgendering him according to thousands of years of biology. That's the only road I'm going down, science.
I can point you to thousands of books that will confirm his gender.
I'm afraid that his change of gender is in his head only, and judging by his outfit at the weekend and music video I'd say that's a pretty troubled place right now.
Have you ever looked up the definitions of sex and gender? Sex is biological, based on whether you have male or female reproductive organs. That's what your books will talk about (although that's actually a bit more fluid as well, surprisingly:
article from Nature). Gender is cultural, it's whether you identify with the female or male role in society. So yes, it's absolutely in your head, that's what gender
is: your self-perception (and not as whimsical thing, I might add).
Taste in clothes is something else altogether.