I think the term No Deposit is cleverly used by Sherbet in order to attract more gamblers as people get excited that they can get bonuses without depositing. I am sure there must be many new signups seeing this promo of the No Deposit bonus.
But the no-deposit bonus is actually given to you by friends, so it not only aims to attract gamblers but also helps gamblers to try a game without having to make a deposit first.
This is a good form of promotion by Sherbet because not all casinos can provide this kind of bonus, right what you said there must be a lot of gamblers who register there to get the no deposit bonus provided by Sherbet.
Even though these bonuses may not necessarily provide benefits, at least by being able to test a game, gamblers can see and feel confident when they play using the money they deposited.