The walking foot acts as an upper set of feed dogs, working in time with your machine. When using a walking foot, fabrics feed more evenly and are therefore less prone to creeping and wobbling about as you try to sew them. And before you ask “What’s the harm in a wobbly seam?” the answer is: a lot. By “wobble” I mean “be totally offset and crooked and very messy indeed.” Sure, your fabric is ostensibly sewn together, but it’s going to hang terribly.
They’re not cheap, alas, and it usually takes a couple of ruined velvet seams before a seamster decides to bite the bullet and make the investment – it certainly did in my case. Nor is there a one-size-fits-all walking foot universal to all sewing machines, so you might not be able to borrow one from a better-equipped chum (although you might – always ask around!). If you go to a reputable sewing supply place with the the make and model of your particular machine, they’ll set you up with the correct one. If you can bring your machine with you to the store, the staff will probably be glad to show you how to use it, too. Be prepared to spend $90 – $150.
If you can’t afford a walking foot, then you can try your luck with a rolling foot, which is better than nothing. But if you’re planning to make heavy use of slippery and/or piled fabrics, you won’t regret the purchase.