They function as an acquisition channel, since each one brings a pre-paid stranger into the practice and a share become regular clients. Making them purchasable online without contact, and promoting them ahead of gifting periods, is what makes them work.
They function as an acquisition channel rather than a revenue line, which is the useful way to think about them.
Each certificate brings a stranger into the practice who has already been paid for, arrives on someone else's recommendation, and has no price objection. A share of those become regular clients, which makes the effective cost of acquisition very low.
What makes them work:
Two cautions. Set and state an expiry policy in line with your local rules, and track redemption, since unredeemed certificates are a liability rather than revenue.
Ask for the rebooking at the end of a gift session as you would any other.