The benefits of hiring a face painter are
You have someone who is experienced in painting children, so they will be swift with their painting-great because children don't tend to sit still for very long.
No need to worry about getting the right paints, brushes and sponges etc. Your face painter should come ready with their own kit, not to mention coming up with simple yet effective designs for face painting. It may look simple, it may be simple - but it requires A LOT of practice.
A qualified face painter is aware of hygiene, meaning keeping the all their art supply clean and keeping the children in your party from infecting each other.
Usually, the price you will have to pay for one face painter to do a party (in average-one classroom) will not be that much higher than if you bought all of the art supply yourself and perhaps taking a short course.

Face painters are usually working all year round and are available for hiring even on quite a short notice. However, it is always best to book in advance.

It's a potential risk to hire a painter who is not covered by public liability insurance. Public Liability Insurance need to be held by the face painter to protect us as well as our guests against accidental injury or damage to property, brought about by them. Should the unforeseen arise, their public liability insurance cover will cover legal fees, costs and expenses. Verify that the face painter you intend to hire carries public liability you that they are insured.

Never hire a painter whom does not have clean equipment and professional face paints which meet the relevant skin regulations.

When booking via a face painting business or a children's parties company, they will make sure to have a standby replacement face painter ready at hand. That being said, normally companies do not have full portfolios of all their face painter but only a taster of the work they usually all do. If you are looking for a specific standard it could be beneficial to look up face painting business of individual, self employed, face painting artists.