I think that's because it's probably considered one of the most basic methods. It's simple: your training loads are expressed as a percentage of your maximal lift: what you can lift once only. That's one hundred per cent.
Alternatively, and this is a much, MUCH better way to do things, you work up to a set of five reps at a weight that is hard but lets you get five clean reps. That weight is your TRAINING MAX, and you calculate your training loads as percentages of that training max which is considered to be one hundred per cent.
The advantage of using a training max instead of am actual max is this: your strength fluctuates from day to day. It depends on many factors including nutrition, sleep, mood and health. If you base all your percentages on a true max, on a bad day even 80% may be too much - because on that day, you aren't close to being able to move that true max. In that case, 80% of your true max is probably closer to 95% of what you can move on the day.
Now, when you use a training max, this situation will probably never arise because your percentages are based on a load you can move for five clean reps. Even on a day where you got dumped, fired, kicked in the crotch and had your tax audited, you will be able to move that at least once. You will definitely be able to hit your reps at whatever percentage of that training max.
Generally you only use those percentages for your main lift and accessories which are usually a big compound lift (squat, press and deadlift variations). For other work, you generally wouldn't worry too much about percentages and just pick a load you can do the required reps with and gradually add weight or reps over time.
Particular percentages are considered better than others for achieving particular goals, typically in specific rep and set ranges. An excellent primer for this is Prilepin's chart, which you can Google; and there is also this:
Very, very broadly:
50-70% for five to twelve sets of two to three reps: speed and explosiveness
65-85% for three to ten sets of five to fifteen reps: size, some strength especially closer to the top end
80-95% for three to eight sets of one to eight reps: strength, with decent size at the lower end for more reps
Honestly, though, if you don't want to think too much and just get familiar with training while getting bigger and stronger do this:
Pick a weight you can do for eight reps for one set. Then do five sets of it for five to eight reps. When you can do eight reps for all five sets, add 10 lbs and start again.