What is YJ Water Pump and Why Do We Use Them?

Running warm at idle, so I thought I'd try a new pump. The old rusty one is what I took off and the new one is a Gates. The counter man at NAPA swears he sold me the right one, but I'm not sure about that. The engine in my Jeep is a 4.2 with a casting date in , probably the original engine, it has a V belt. Which pump is correct? Thanks!

So, the coolant was going the wrong direction in the engine? The guy I bought it from had a lot of engine work done and most of it needs redone. I did some research on line, watched some videos, but came up with conflicting data. There seems to be lots of confusion and misinformation from the manufactures on the 4.2 water pump. I did order and install a new pump for a , V belt 4.2, I received Omix # .13, the vanes went the same direction as the old rusty one I took off, so I assumed it was the right one. Still had cooling issues, and I wasn't happy with the poor casting of the Omix pump, so back on line I read that the correct Omix pump for my Jeep is .04. So, I Ended up going to NAPA yesterday to look at a GATES pump, that's when I discovered I may have had the wrong pump the whole time, I bought Gates TWP for $35.49. As a side note, I had to grind the casting on the Omix pump to clear the timing cover, ( wrong pump clue and I missed it) run a drill thru a couple of the bolt holes, heater hose outlet too large to attach the 3/4 hose. The heater hose outlet on the new Gates, easily accepts the 3/4 heater hose and the casting looks very nice. I'll install today and post the results. Thanks for all the help!!!!!!!!!