Or so he could constantly play below his skill level as the matching system would think he was worse than he is.

If he was doing that on ladder, that’s generally considered to be bad form.

If I remember correctly, there was no matching system in BW.

Brood War had (has) a ladder system and then there were external leagues such as ICCUP that has ranking as well.

There's a term for this: "sandbagging". It happens across lots of rated competitive platforms, from pool to chess to Scrabble to stocks...

Haha I remember my dad teaching me this term during sports teams tryouts, the kinds where you would be drafted onto a team at the start of the year. I was a decent player, but definitely did not try my hardest during tryouts -- if I had, I would have been the worst "first pick", making my team bad.

Game theory to 9 year olds?

It was also because in the Battle.net chat rooms, your icon would have a higher number if you were ranked lower.

So if you wanted to be rank 9999, you just forfeit all your ladder matches.

When I used to play there was rampant abuse of the system by script kiddies who were using hacks to increase their latency to get auto-dropped from the game, which wouldn't count as a loss.