The object of 501 and 301 is to double out. Beginning players don't think about their out until they are at 40(or even lower); and most average players don't consider their outs until they are well under 100.
Undoubtably most players have seen out charts posted where they play, but many outs these charts don't represent real world situations.
Many out charts rely on the triple 20 for the first dart, simply because this gives you the maximum score. While extremely high outs necessitate hitting the triple 20, most players would be surprised at how lower outs(but still above 100) can be taken out by utilizing other numbers that, once they are presented, make more sense than simply going for the maximum score on the first dart. We would all like to be able to nail that first triple 20, but what if you're just high or low? With most traditional out charts you are then faced with another 3 dart out.
As an example, consider taking out 122. If you hit the triple 20 on the first dart, great, you're left with 62: you're left with many choices, but none are statistically desirable. One way would be single 12 and then double bull. Or maybe triple 10 and then double 16? Consider what happens if second dart misses: no way to double out.
Mastering out shots requires the thrower to take into account the possibility of missing the target of the first dart. This is certainly not any excuse to think that you can't hit that triple on the first dart; it allows for the (unfortunately) inevitable possibility of missing that triple, and yet still leave the thrower with a 2 dart out.
Consider our 122 again. Throw at the triple 18; if you hit it, great dart! now you have 68 left. Sure, you now have no cake walk with 68, but now look at what happens if your first dart had been a single 18: you would be left with 104. While it's a big two dart out(T18-DB), it is a two dart out. Had you missed the triple 20 and hit a single 20, you would have 102 and there is no two out for that!
It is important to understand this out strategy: in a close game, one dart can be the difference between winning and losing. An understanding of out strategy will allow players to improve their ability to set up their outs and thus improve their game.
Please use this link for a more detailed explanation of these out charts. You will get a real understanding of what allows good players become great!