Sexuality's very complicated but also really simple, depending on what aspect is actually being talked about. The way I see it is that sexuality includes so many differences, and labels such as 'pansexual' address those differences (if someone so chooses to apply it to themselves). But then there's 'sexual orientation' (gay/straight/bisexual) which isn't so much about those nuances, but quite straight forward, in that it ignores the many possible reasons
why someone is sexually attracted to someone else, it's just that they
are attracted. It's not what leads to a sexual act taking place, it's who the sexual act is with. So in a way, 'pansexual' is more a descriptor of the individual, a statement expressing their wider views on gender identity/nonbinary genders, and also that they want people to know what those views are. It might be that one person wants to do so because they think it sounds superior to just saying they're 'bisexual', or it may be that they want people who identify with non binary genders to feel more validated, there's a whole load of reasons why someone might want to associate with a label. Orientation isn't an expression of beliefs or attitudes though and is simply determined by the resulting sexual relationships. And labels regarding orientation I think are really quite important since they have certain legal protections associated with them which we had to fight for, and is another reason why I think it's useful not to get muddled up between orientation and other preferences/identities/nuances of sexuality etc. Those laws don't see gender identity (and neither does the marginalisation of gay and lesbian people), they're specifically about the sex of the people you sleep with.
Put simply, you can tell if someone is gay/straight/bi/asexual based on who they do or don't want to have sex with. You can't tell if someone is pansexual based on that. And that's because it's a description of that persons personal attitudes within their sexual orientation, rather than an expression of orientation itself, which is what bisexuality is. But yes it's still valid for what it is.
As for whether or not labels like pansexual are important. They're important if the individual using them to describe themselves considers them important I suppose. They can have certain social benefits if you want people to know what your 'type' is, or as a way of sharing something about your views. If someone is using their label to put someone else down then they're just a twat tbh

But I don't think that's necessarily a problem with the label... some people are just twats.