We have not received permission from worddog to share the quoted post.
As I see it, it's to let people have certain reputation on certain chars. Players are supposed to have their own stalls and sell their services to others, be it crafting or processing. If players start to do everything that kind of reputation gets diluted and if you can't setup several stalls from your alts (I'd expect that you can't) then in the current system your "account" would be mainly known for just one thing. And then if you wanted to change that reputation, you'd be able to just switch your stall from your main to your alt.
The biggest example of this that I know from experience is several different crafters that I knew by name in L2 because one of them was only crafting mage gear, one was making phys gear, another weapons and another one jewelry. And if their stalls weren't online it'd usually mean the player themselves were online, so I'd just PM them and ask to craft smth. I expect to do the same in Ashes for different services.
This kind of approach distributes money to more players and promotes communications and socialization. People who have the alts could still make stuff for themselves if they need to, but that's where the usefulness would mostly stop.
Also, having gathering professions on your alts would allow you to avoid guild wars standing in the way of your farm. This also comes from personal experience. If I wanted to just go gather some resources in L2, I'd just have an alt that wasn't in the main guild which was always at war with several other guilds. If I could gather resources on my main - I wouldn't be able to do that.