If you want to use these skills not only for yourself, but to sell them to others as well (as labor), it's really helpful to know where you stand on the market.

If you want to use these skills not only for yourself, but to sell them to others as well (as labor), it's really helpful to know where you stand on the market. Otherwise you might think you're good enough at something, devote a lot of time to it, but only later learn that others are better at it. In fact, I think it's quite common scenario in a lot of solitary pursuits, be it intellectual or artistic. Without the immediate ability to compare, you can spend years doing something without being rewarded for it at the end - for example, Hitler spend a decade trying to be a painter, before he finally owned his failure and switched to politics. If there was a way to get reliable feedback in painting without putting a decade of work into it, he might've had abandoned it much earlier and not grow his resentment towards "the gatekeeping Jews".