The straight ally flag isn’t saying we are oppressed or part of the LGBTQ community, (That’s what the straight pride flag is) it’s more of a symbol saying I’m here for you and I’ll support you. It isn’t a pride flag.
As Sin-69 said, it’s a way of saying you can trust me I’ll support you.
I personally bought a straight ally pin so people I meet know I won’t judge them for being non-binary or bisexual or whatever part of the lgbtq community they are from