I've applied the label to myself before. I used to identify as 100% lesbian, but then I had the experience of dating a trans girl and a genderqueer person. Those two relationships are some of the most meaningful I've ever had.
I would have felt comfortable calling myself a lesbian when I dated the trans girl, because after all, she identified as a woman and lesbians are by definition "women who are exclusively attracted to women." However, getting serious with someone who was genderqueer and who goes by male pronouns about half the time made me think about my identity. I know I'm not bisexual or pansexual because I've never been interested in dating cis men. So, sometimes I call myself queer.
Unfortunately, I've also seen girls who are basically bisexual identify themselves as "queer." I've even seen some straight girls call themselves "queer" in an attempt to show support for the gay community. I don't identify with either of those groups, and that's why I take issue with the word "queer." It's too vague, to the point where it doesn't actually have much of a meaning.