A planer is a worthwhile investment if you're serious about cabinetmaking, as it's useful for creating smooth, evenly-thick boards.

The jointer is used to flatten one face and square up on edge.

The planer is then used to make the second face flat and parallel to the first. The planer can also smooth rough stock and reduce stock down to the exact thickness you need.

You don't need a thickness planer to get by, but it's nice to have for larger jobs where you need to replicate parts accurately. With a thickness planer, you have more control over your stock thickness than with any other tool.