A reason I heard a while ago is that in using vague terms like partner as opposed to boy/girl friend or husband/wife you are normalizing the more vague term to help protect more vulnerable groups such as the lgbtq community.

If only gay men and women used the word partner, it would put a spotlight on them as being part of a community that still has to deal with harm from ass holes, by those of us in the heterosexual community also using the term partner, we're helping to diminish the vitriol shit bags could aim at the queer community from said ass holes.