The most basic function of any easel is to provide a safe spot for your canvas, or support, during the painting process. It's important that the canvas be held at an angle you prefer and that allows you an undistorted view of the picture plane.
Most painting easels will have at least one and some will have a variety of trays and drawers where you can store paintbrushes, paints and a jar or two. This is a space-efficient way to paint, allowing you easy access to the tools necessary for the painting process.
It is a common practice for artists to tape papers directly to the outer edges of the easel. This allows you a clear view of photographs or paper studies you have made of a subject in preparation for a painting.
Some easels are meant to stand on the floor. Others are designed for tabletop use. This is important if you prefer to paint standing or sitting down. There are also easels, known as French easels, you can fold up and take with you when you wish to paint outdoors or away from your studio.