why is there a "+" in lgbtq+?

Because the LGBTQ initialism is quite sufficiently ridiculous and illogical to start with, so it doesn’t make any difference what anyone decides to add to it. And since it’s not protected by any copyright, anyone can add anything that they fancy. They can also invent their own, e.g. XGTR, standing for "ex-gay, trans and reincarnated".

It's just a way of being more inclusive of all sexual orientations without having an excessively long list of letters.  It includes asexual, pansexual, and demisexual to name a few.

So LGBTQ stands for Lesbian, Gay, Bisexual, Trans, Queer/Questioning. But there are so many ore sexualities and gender identities, so it makes it a lot easier to just write LGBTQ+.

It's easier to write LGBT+ than LGBTQQIAAP.

Lesbian

Gay

Bisexual

Transgender

Queer

Questioning

Intersex

Asexual

Allies

Pansexual

No. There are so many different identities in LGBTQ+ that it would be insanely long. The + is there to show that people who are something other than gay, bi, lesbian, trans and queer are valid. For example, nonbinary, asexual and pansexual people are part of that plus. It shows inclusion and acceptance for everyone who isn’t straight and/or is the gender they were assigned at birth. 

Because it misses relevant groups out such as asexuals.

Its shorthand for LGBTQQIAAP2S or whatever the full version is. Its much easier to just use LGBT+ I don't even think there is an official full acronym. I googled it and everyone says something different. 

LGBTTQQIAAP, lesbian, gay, bisexual, transgender, transsexual, queer, questioning, intersex, asexual, ally, pansexual.

and the rest...

Plus whatever else you want to think of yourself as - maybe a goat/human or something?

At least we can go back to calling them queer since they have that in there.