What On Earth Is A Clean Out?
So, what exactly is a clean out?
A clean out is a cap placed on your plumbing system, usually near your fill lines if you have a septic tank ; For city sewer systems the clean out is usually placed on your main line near the water meter. Clean outs look like the one in the photo below:

Why Do I Need A Clean Out?
To be 100% honest, you don't need a clean out installed. However having a clean out installed is not expensive and will certainly save you money in the long run. Now you may be wondering how having a clean out installed will save you money? Having a clean out installed will save you money in situations where your main line gets clogged. For those who don't know what a main line clog means, your main line is the main pipe that runs from your water meter to the house then spits off to each area of your home in which you run water or sewer. This means if you have a main line clog everything from your sinks and toilets to your bath tub will back up and clog. This type of clog can get EXPENSIVE however installing a clean out will make it much cheaper.
How Will Having A Clean Out Installed Save Me Money?
When you have a clogged pipe without a clean out your plumber has to pull out your toilets, snake the line, and re install the toilets. After all this, there is absolutely no guarantee that your plumber will be able to reach the clog and in this case you will still be stuck with a high bill and no relief from the issue in which you called your plumber for to begin with. If you would have had a clean out installed, your plumber could have simply popped open the clean out cap snaked the drain and reached the clog with little to no effort compared to the massive work load I mentioned before. The less work your plumber has to do the less you will have to pay. The harder and longer your plumber has to work the more you will have to pay.
So, to sum things up, installing a clean out is VERY recommended, will certainly save you money and is not very expensive.