I'm currently looking for somewhere to live with a friend. I've seen "no sharers" on ads a few times. What exactly does this mean? Single people/couples only?
If it is, what's the reasoning behind it, and do landlords really expect people to leave a room vacant (or even 2 rooms on some ads)?

I suspect they may have bad experiences with people who share and assume that they are less likely to be model renters - staying for ages, paying rent on time and not burning furniture.

They would be happy if you both people signed an AST so that you and your friend are "joint and severably liable" i.e. if one of you does a runner the other is liable, otherwise it is separate contracts potentially making the property a "multi occupancy" which is mega regulated or a lodging arrangement which is totally unregulated.

The LL wants to be protected and sharers have reputation for dodging liability. If you are happy to share with friend for the duration of the contract and you trust each other then sign a regular joint and severably liable AST and you will have little aversion to sharing...