Think of it this way.
Scenario 1: your favourite escort is back in town and you're set on seeing her. She gets inundated with calls/text/emails, you and everyone else wants to see her on Thursday at 6pm, but you've sent a deposit to guarantee your time with her. You're making yourself priority, and showing her you respect her time enough not to waste it. You are penned in her diary for your requested time and date. You even have less cash to carry on the day (bonus points for legalities, right?).
Scenario 2: your favourite escort is back in town and you're set on seeing her. You call and request Thursday at 6pm and she books you in. She's turned away several other clients, because maybe you were the first to call or confirm the booking. You cancel on the day (or worse - no show), and now she's lost that potential income, and it's too late for her to make it up by seeing someone else. Now she's upset, out of pocket, and you're in her bad books.
The obvious problem is a lack of trust from both parties. But a) why would you want to see someone who you don't trust and feel is professional?, and b) what reputable escort would really risk their reputation over 20 quid? Especially when she can make the full amount from actually following through on her commitment and seeing her client? Don't look at is as someone trying to 'scam' you, but as a woman trying to run her business efficiently and with financial security, and clients should see it as making themselves a priority and establishing trust with a provider.
Everyone will have a different approach when it comes to deposits. Some women will take them for new clients, and won't ask again after she meets you and knows you're upstanding. Others will ask every time just for peace of mind. And maybe some don't ask at all, but if you screw them over, good luck trying to book her ever again. Unfortunately, there are escorts who will take advantage and I'm not at all defending them. They should be flagged and avoided. But mostly, we're just trying to guarantee some stability in an already hectic job and lifestyle.