Because they are softer and won’t damage the steel parts. And/or tools with protected tips for the gold items such as the rotor.
After having made some marks on my own watch, I got a scolding from my mentor and sent off with a set of his brass tweezers and a command to get my own.
I’m sorry this happened- it doesn’t detract from the movement’s performance but certainly spoils the appeal of the watch. I’d return it and start over with another. Because you might always be wondering and looking to see if something was damaged that you have missed.
Cazalea