If you have a pair of Yakima bars, you need to get endcaps. Besides giving your bars a nice finish look, they help prevent moisture from getting inside the bars. The last thing you need is rusted bars, which apparently is NOT covered under Yakima's warranty. Do yourself a favor and plug up those bars.
These are nice to have on your bars to keep water and other debris out of your rails. That being said you do not need to buy these if you are buying the railgrabber also because a set of four are included. I learned the hard way. No biggie I just payed like $8 more than I needed to. Just read about the product or ask a rep before purchasing both.
Unless you use some other accessory at the end of the bars you need these. You can live without them, but to have a yakima rack system and unfinished bar ends would be silly. Without these the ends are rough and ugly. And without these you will probably get some stuff and moisture inside the bars. You can live without them if you like but it would be silly.