Why does it bother you if someone 'tells you what they think they are'?
Like I said...it is most common in certain contexts, like education, healthcare, etc. where gender identity is relevant. Your banker, barber, mechanic, store clerk, etc. likely don't care or need to know...and it will almost surely never become 'common' to put pronouns in email signatures for the vast majority of people...but for educators, counsellors and healthcare providers, people in social services, etc., and simply those with a big platform who wish to show support, it can be relevant.
The use of pronouns in that manner amounts to visible representation (by those who are trans) and support (from non-trans people) and has been shown to not only raise awareness, signal understanding and indicate a positive environment....it can actually save lives. There have been many cases where the existence of passive visible supports, even without any direct interaction between individuals, has been enough to prevent teen suicides, for example. I could provide you with examples...but I'm guessing you're not interested (which is fine).
It's perfectly fine if this has no bearing whatsoever on your life...and no one is asking you to take some major interest in it either...but instead of 'just laughing' or find something 'stupid', why not take a moment and ask yourself "I wonder why they do that" and consider the issue from the perspectives of those for whom it apparently does matter.