For the most part, put them out of your mind entirely, because they usually are more expensive than buying business class, especially when buying tickets in advance.

If you’re planning to purchase a business class fare, and there is PN space, and it’s a CPU-eligible route, then it’s worth it to do a quick search for YBM fares to see if they’re cheaper, because UA has listed inexpensive B fares in some markets that are intended as a way to sell Economy Plus (people flying in B get E+ access for free). The other place you might be able to get an instant upgrade is if you’re buying a Premium Economy fare overseas; the connecting domestic legs will be in B.

That said, 9 times out of 10, you’re better just to buy the seat you want and move on.