While I don’t think that it’s “pure guesswork”, one does have to apply some basic assumptions, which may or may not be correct, so I’ll grant you that.

For me, it’s about the privacy: anyone can see both the inputs and the outputs of any given transaction and as such could build a picture of what coins are being controlled by which wallet addresses – I no more want anyone knowing how many coins I control, than I would want anyone knowing what my bank account balance is.

I think (but I’d have to check) that ‘wallet clustering’ is about the tell-tail signatures that wallets have; that is to say that any given wallet will construct a transaction in a given way. I (like so many others) use a particular wallet. If you read my posts, you’ll very easily discover which wallet that is, (or one of them, least ways) so that’s just another bit of intel that could be used to track me. There are ways around that and I do use a mitigation technique which, together with so-called ‘mixing’, would make it very difficult for anyone to trace which coins I control. The idea is to create enough confusion and doubt about which transactions are mine and which are not, that it becomes too time consuming (and as such too expensive) for any agency to pursue.