We can never know if anyone truly understands, fully grasps what it is we are telling them. I've often wondered if it was just an illusion that people know me and know what I've shared with them.
I think unsolicited advice comes from a place of "Oh he's talking to me, he must want me to solve something." I think we all feel like fixers to some extent. Some people way more than others.
When someone vents the only way to not offer advice is to process the communication and say "I hear what you are saying." Which can also come off as disingenuous. There's no way to win, I guess. Sometimes when somebody doesn't tell me how they relate to what I've said, I feel like they haven't heard me at all. Maybe I'm just annoying!
I'm pretty sure what I do at therapy is venting. So I won't say it's useless