A brush rest is a minor detail in the encaustic setup; but, it does hold a purpose. Granted you can get away with not having one, and if you are using a kitchen griddle you may not need one because griddles typically have a raised edge that can act as a rest.
The purpose of the brush rest is to keep your brush handle off the palette where it not only keeps your brushes from getting hot (a safety precaution), but keeps them out of puddles of paint. Additionally, palette cups tend to be to short to hold the brush up by themselves; so resting a brush against something comes in handy. My modified coat hanger works well for keeping my brushes off the palette, from tipping my paint cups over, and because there are two parallel wires the brushes stay warm and off the palette; better than setting them off to the side where they cool and require more time to soften to a workable state. Also the spring action of the wire makes it easy to remove when it gets in the way.
A quick tip: Instead of using a rest it helps to cut down the long handles of your brush to a more desired length so that your palette cup will be less likely to tip over.
