I disagree with this mindset completely but it's too much to get into detail with.
But basically "LGBT" gives us an easy thing to identify with to fight for equality, reflect on history with, for being direct with language, etc., but it's not something that exists as a community like I think your mom imagines. Gay pride parade =/= The LGBT community.
Talking to other gay people (thinking of a few posts in here) doesn't make you isolated from everyone else, that's silly. You can talk to anyone you want, if someone doesn't talk to you because you talk to other gay people, that's on them.
Likewise, a gay pride parade is something someone does at absolute most a few times a year if they travel, but that doesn't separate them from everyone else the rest of the year. That's on people like your mom who are eager to "other" them instead of looking to welcome people who aren't like herself in some way. Which is incredibly short minded. "We're isolating ourselves attaching ourselves to a community that's seen as different to some." I can't help but groan at that, I don't believe all LGBT people are unique and all that, but this point just seems like "try to never stick out so someone can never see you as different" which is boring, way too much of hassle to try, but more importantly stressful and harmful. (And my second point below.)
But I think instead of really getting more into this I'll just do a quick bullet list of the first things that pop into mind on why we need community (in the broad sense and maybe more specific.)
- Truvada. If it wasn't for LGBT activists and the community that wants to prevent HIV and promote safe sex, it would have had the negative connotations that plenty tried to falsely attach to it stick and been pushed aside. It's very easy to just go, "Safe sex! Boom, no HIV", but that just hurts people and isn't effective. Straight people push it aside because it reinforces the whole gays are *****s idea, but this shouldn't be a moral judgement, simply this would stop the spread of HIV - that's good.
- Everyone benefits from people challenging norms. Many LGBT activists challenge gender norms, how we teach gender to children, how we're expected to act, etc. That benefits everyone.
- Back to safe sex, you need pockets of LGBT people somewhere (even online) where people can learn about safe sex. Safe same sex practices aren't going to be taught in school. [And in the US, many schools still teach it's not okay to be gay. Those kids need somewhere to go, again, even if just online.]
- The T part needs community so badly, probably more than any other group. Wanting them to not have a community would be disastrous when they already aren't able to easily get information and help they need. We need things like trans clinics because straight/cis doctors won't prescribe puberty-blocking medication or hormones for trans youth because they aren't comfortable with that. Imagine that, just
nah, I don't like it, I won't give you the medication you need.

This is where the "LGBT community" needs to step it up more and spread how much this is an issue. Cis people won't suddenly care.
- We needed many LGBT individual a few years ago to be as vocal as possible about who they are in response to gay youth suicides to try to change something and to form a community that gave kids hope. I can't imagine how many children watched those videos who didn't know what they were or even know a gay person.
- You need a LGBT community to watch for how it's going to be remembered. If left for the masses to decide how our history is remembered (in the way that's most favorable for the general public) you get things like the Stonewall movie which completely changes history to something that's not accurate.
- Lastly, mainly for fun, you need gay bookstores because half a shelf in a general store is only going to have the greatest hits and bookstores won't host events with select appeal. Really ignore this one and take away that having an LGBT community is so important for health issues that wouldn't otherwise get attention. There's more than just same sex marriage.