I have never made any date requiring a deposit but that won't stop me from tossing in my non-refundable 2 cents.
Your main Q was about the deposit and its relation to the total fee. That is usually made very clear in her ad or can be clarified during set-up. The total amount is for her TIME and TRAVEL and she might ask for "50% deposit" or "$X deposit"; you know the total so you pay half now and half the day of or $X now and $(total - X) the day of.
I don't recall any ads that break it out as "TIME only" = regular hourly fee; "DEPOSIT REQUIRED" to cover other expenses, but that, too, should be obvious or clarified via email or phone.
A Provider incurs certain costs in the course of setting up any meeting and some of those must be paid whether or not the party of the second part shows up. So, for longer dates or travel dates and some other cases, the Provider wants to make certain that her effort and expenses are paid in advance with a deposit. You can sort of guess along with me which expenses she wants guaranteed to be covered:
new outfit - no, she can use that on many other occasions and recover the expense over time.
2 hours getting ready, makeup, etc. - that's 2 hours NOT used for some other purpose; lost opportunity cost; some might factor that in to a deposit, some might not.
hotel room - used or not, the hotel must get paid - USUALLY asked for as part of a deposit. This is an actual $ expense. (No one adds up the cost of shampoo, soap, dry cleaning, manicure, ... to get an exact dollar cost of "getting ready" but there is a feel for it.)
air fare - if travel is involved, it is also often requested as a deposit before agreeing to set a foot out the door. If she is a touring Provider who normally travels to that city and books clients to cover those expenses, then travel is built-in to her fee. Air fare or travel: dollars out of her pocket that she wants to guarantee.
Consider your own preparations for a meeting and costs involved. ("I left work early and drove two hours ... ") You can keep adding up the costs and that's why some Providers ask for a deposit