Fluid gender (identity; ascribed gender is externally imposed and to the extent it is or is not fluid, that's on society not the person to whom gender is ascribed) is an aspect of some people's gender identity, it is not something people (at least, many people) believe is inherent with all gender identity. Many trans people have as fixed an identity as the most cisnormative people believe all gender is, it's just not a cisgender identity.

> Is it a necessity?

For some, not for others.

> Then why adopt distinct male or female features, genitalia which do not work as intended?

There's a number of reasons (including the way society ascribes gender and mitigating the clash between ascribed gender and gender identity), which vary from individual to individual. But, also, that description of genitalia is the opposite of reality—for people that choose to change their genitalia, it is specifically because the new ones work more as intended by them than the old ones, not that they “do not work as intended”.

> Sadly, I don't know any trans person to ask them how they feel and why they feel that way, in person.

Happily, there's lots of information publicly available from trans people addressing those issues in every kind of media so that you don't need to rely on pestering people face to face to explain their gender identity and feelings surrounding it to you, which lots of people don't like to do.