I added pronouns to my bio a couple of days back.
As a cis het woman, my gender identity was never in question. Why, then, did I feel the need to do so?
It was to show solidarity with non-binary and transgender persons, but it was also more.
To step back a bit...
Most of us have grown up thinking sex and gender are the same. But they are not.
Sex is what is assigned to you at birth based on your sexual organs.
Gender is what you identify as, regardless of what was assigned to you. Gender, unlike sex, is non binary.
There are non-binary people who do not identify as either male or female.
There are transgender people who's gender may be different to the sex assigned at birth.
Their preferred pronoun may not be obvious just by looking at them, which is why, to avoid being misgendered, they specify.
Why then, should a cis-ally add pronouns to the bio, when the gender is "obvious"?
It is essentially to display a symbolic solidarity. To tell non-binary and transgender people that they have our support, that they are normal. And to prevent misgendering.
But there is more.
Non-binary and transpersons are often the target of hate. By putting up pronouns in the bio, cis people destigmatise "otherness" and help create a community where it is harder to identify a non-binary or transgender person from a cis person.
Putting our pronouns in the bio is a small thing we can do to make non-binary and transgender people feel more accepted, and to destroy myths around gender identity.