Anger usually stems from a boundary being violated. It's our instinctive evolved response for pushing back against someone who has encroached on us.
When we "vent" to a peer, it is relieving because it transfers that personal boundary to a group boundary. We feel "OK, my peer will now back me up the next time this happens."
Venting is like yelling for reinforcements when you see someone storming your corner of the castle wall.
You'd still vent, if alone on a desert island, maybe to The Gods.
Ever hear yourself saying out loud, usually with expletives, something that you would "never say"? Listen to that voice!
That's communication of unconscious but frustrated thoughts. Sometimes it has to come out verbally because ones inner dialogue isn't strong enough due to social or superego suppression.
After you said it to yourself you feel better.
We are not the singular identities simpler minds suppose us.
That's not what it's for. It's for getting thoughts out of your head, having the opportunity to articulate strong feelings that might otherwise be fuzzy, and for feeling heard and supported.
If changing your opinions is the goal, venting is pretty clearly not the way to get there.
But I do wonder how much of it actually lands for the recipient. As an influencer putting out a carefully curated image of yourself, how meaningful is it to be boosted by a bunch of mostly-anonymous strangers? Wouldn't it just feed into a kind of dissociative thing where you recognize that they're praising and lifting up a mask and have no idea about the particulars of the real person's struggles? I don't know.
Venting isn't for catharsis; It's for seeking validation. When you vent TO someone and they lend a sympathetic ear, it's a huge help.
Venting is a social bonding ritual.
You generally don't get this online because most of the audience is NOT sympathetic, so you'll get criticism after criticism, making things even worse.