Hi Yusuke! It's an expression used in so many ways that it's tough to nail it down to one or two. The speaker is making a statement of speculation about something that you look like you might want or need. It's a guess! You look like you could use a rest. You look like you could use a friend. You look like you could use a vacation. You look like you could use a day off. You look like you could use a drink. If you want to you can answer the other person and tell them that they hit the nail on the head. Then go out for a drink or whatever. Or not . . .

It means that it seems like 〜 would be of benefit to you.

I think it depends on context. Mostly it is an comment expressi empathy, but it could be an offer. For example if someone says to you, "you look like you could use a coffee," it suggests that you look tired and the person has noticed. Are they handing you a coffee? Are they offering to get one with you? Are they just saying it to be nice? That will depend on context.

It is the same like saying You look like you could do with ……… Yes it is a suggestion.

It is an uncommon way phrase that isn't used often. It can be literally used to offer advice but depending on the context and how or when you decide to use it can be a condescending way to poke fun at someone.