You are getting prossies and civvys mixed up mate I want to fuck for cash and walk away not have them cook my dinner.
You are paying to be able to leave at the end with no chance (usually!) of emotional attachment, truly 'no-strings'. Whether the lady is 18 or 80, a civvy encounter has all the possibilities of one party (or both) wanting to further the relationship, and that is messy, especially if you are in relationship already! With a WG, it's highly unlikely she will phone you the following day asking you for a second "date"