OH BOY! This is THE MOST important part of preparing a solder joint. When I worked as a plumber at Purdue University they had the COPPER ASSOCIATION come and do a hands on training session. When water passes over this ridge left in the tubing a phenomenon known as "HYDRAULIC JUMP" occurs. I have seen this on other plumbing forums that use multi syllable mumbo jumbo words. Water passes over the ridge and causes the water to swirl. We saw an example. It looks like backward horse shoe prints that get worn in the pipe to the point of causing a leak usually within an inch or two of the fitting. When I took the Indiana State Plumbing test if you did not ream the tubing it was an automatic fail. As far as I know getting a plumbing license in some states is as easy as going to an office and doing a computer test. Not Indiana. You still have to pour a 4" lead joint. But it has been a few years now so that may have changed. Also on the Indiana State copper test you have to figure a 45 degree offset with a 90 and two 45's. Your math is graded. Then your solder joint is cut and peeled back and graded. The 4 inch lead joint is capped at both ends and then about 40 pounds of air pressure is applied. It is put in a tank of water. If it does not leak then they bust the bell open and the lead joint is graded. OH, if it leaks you get to go home. Some State plumbing tests are VERY STRESSFUL! One of the guys I work with now at ASU worked for a plumbing contactor who told him "IF I CATCH YOU WASTING TIME REAMING THAT COPPER PIPE I WILL FIRE YOU!" Ain't that just great! I was taught to ream pipe long before our hands on seminar but it certainly proved to me why it is so necessary to do it. I also really like to clean pipe and fittings with those green Scotchbrite pads. They don't slip in my hand like sandcloth does.