Younger people use "queer" to be more inclusive. And, to recognize that typically white, cis, gay men for decades used their intersectional privilege to shape LGBTQ equality efforts to suit their needs more than others under the larger umbrella - think Joe Solmonese (though I doubt most of the younger "queer" kids even know who he is/was or have even heard of UnitedENDA).
Old gay men (and a few not so old ones!), on the other hand, want & need a term that can be used to refer to themselves only, especially when the issues at hand pertain to their lives. Gay is the "G" in LGBTQ.
"LGBTQ" however, is often seen as too cumbersome and not inclusive enough - and adding more letters only invites ridicule. So, younger people use "queer" as a reclaiming of the word, as a term that flows off the tongue much easier than a list of consonants, and because they're proud to invoke the original meaning of the word before it was applied to LGBTQ people, which was essentially, "different, unexpected". They are pushing back against the very conservative, very Christian idea that everyone needs to conform to a single societal standard to be recognized as equal members of society.
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