Of course they're similar. But just like all squares are rectangles, but not all rectangles are squares (did I get that right? lmao), all BL/GL is LGBT+ but not all LGBT+ is BL/GL. I'm interested in more than that (though I do read BL/GL as well!). I'm interested in transgender, nonbinary, bisexual, pansexual, ace, etc. characters and stories too.
Additionally, most people feel that BL and GL imply romance as the focus of the story, but LGBT+ doesn't necessarily have that connotation. I find that nuance useful.
Ultimately, there's lots of LGBT+ content on Tapas, so I think a tag for it would be useful for a lot of folks. But Idk. I just know I'd use the heck out of it.