Suppose you are deep frying dinner and when you are done your wondering whether you should keep the oil or toss it out.
It used to be your mom/grandmother or maybe even you (I'm guilty) would let the oil cool and store it. This is no longer the case.
Once you use oil for deep frying, when you are done you need to throw it out.
The food that you cook will leave little particles, and even if you take the time to strain it, the oil could go rancid.
The next time that you used it you could end up getting sick from the rancidity or the oil could catch fire because of the extra proteins and sugars that are left in the oil along with the basic structure being broke down from the initial use.