You use hot jars so they don't break from thermal shock. Putting the hot food or liquid in the jars, then into the canner can cause the bottoms to fall out.
Most people put the water in their canner, then sit the empty jars in . Take out a jar, fill it, then return to the canner to stay hot until all are filled and ready to process.
That, or use the dishwasher and keep them hot in there. Another way is in the oven, in a roaster pan with some water in the roaster. Never sit in a dry pan or they can break from dry heat.

Ok, thank you so much!!
I made some pickled squash and didn't have the jars hot. I must of got lucky because none broke. Thank goodness! But now I know! My next adventure is to try Annie's salsa from here, when my tomatoes turn red.

I don't heat my jars. I've determined that since they are tempered glass they can withstand the temperature change. When you put hot food into the jar, it gets hot. I've only had one jar break in the last 20 years and that was in a very old quart jar. The newer half pint and pint jars have been reused a thousand times and I've had no breakage at all. The only concern would be if you are canning in a very cold room, which isn't likely with all that heat from the BWB canner and from the food heated in a pot - that's a lot of ambient kitchen heat. I do put my jars on a thick microfiber cloth when I fill them which keeps them insulated from the cold of the kitchen counter. I wouldn't worry about having hot jars, as long as they are washed and rinsed well they should be fine waiting for the time between that and filling them IMO. For me it is a waste of my time trying to keep jars hot when I make up a couple hundred every week all year (for my jam business) and time is money to me.
Nancy

I much prefer to use hot jars. Have had too many messes to clean up otherwise - both in the canner and on the countertop. Perhaps it is less of a risk with jam or jelly since they aren't as hot as boiling liquids would be when poured into jars.

Dave

I know it's not recommended, but I wash my jars and then put them in the oven at 175. But I think it depends on the reliability of your oven though.

My oven is pretty good at maintaining an even temperature. It even has a bread-proofing cycle, so I'm not too worried about the temp getting too high in there. Haven't had any break yet.

I fill the hot jars on a tea towel, to catch spills. So I'm not worried about them sitting on a cool counter either.