What is the point to lots of the higher league players taking their scouting worker and mining from the enemy field? Is this just for BM? What is the purpose of this?
This is to disrupt mineral syncing. Experienced players will try to line up two of their workers to alternate mining from each of the closest mineral patches. So if they have 12 workers, 8 will be mining from close mineral patches and 4 from far mineral patches.
With your scouting worker, you can disrupt this by jumping on these close patches with good timing. This causes your opponent's workers to desycn from the patch and move to another, often starting a chain reaction. This can slow income heavily.
The goal isn't actually to steal minerals, it's to disrupt mining.
I've seen pro-level players use their scouting worker to try and disrupt the efficiency of the enemy's mining. Since only one worker at a time can mine any mineral patch, they will try to time their scouting mining so that it makes an enemy worker's AI believe that that patch is occupied or oversaturated, causing it to run off to mine somewhere else. They will usually do this on one of the closer mineral patches, as these are mined slightly faster.
It's unlikely that this tactic makes a significant impact on the game, even if executed perfectly. I think it mainly boils down to (1) the psychological effect or (2) just something to do with your extra APM in the very early game.