PostEaseA value proposition answers one specific question
A value proposition states, as plainly as possible, what problem a business solves and for whom. Not a slogan, not a mission statement. A direct answer to "why should I care about this, specifically." The most common way one fails is being vague enough to apply to nearly any business in the category, which tells a reader nothing distinctive at all.
Specificity is what makes it useful
"We help businesses grow" could describe almost any company; "we help independent bookstores compete with large online retailers through community-driven events" describes one kind of business with one approach. The second version is more useful precisely because it excludes more than it includes. A value proposition specific enough to be wrong for someone is specific enough to be right for the audience it's meant for.
It belongs in a bio, but it should show up in content too
A value proposition is a natural fit for a profile bio, but it earns its keep when the content itself reflects it. A bio that states one thing while the posts drift somewhere unrelated sends a mixed signal. A reader notices, even if they can't articulate why the account feels unfocused.