I have never used this, but my question is...once you spritz the simple syrup on the cake how long do you wait before crumb coating?

Bump. I would like to know this, too.

I use simple syrup sometimes and I only brush it onto the surfaces that will be inside, like between layers. I don't ever put it on the edges or sides. I use my layers right away. Hope this helps.

Doing it on the insides keeps the cake moist? As I understand that is what it's purpose is correct?

torte, light wash with simple syrup and then filling. one right after the other. simple syrup can be used to add moistness and/or additional flavor.
i use lorann oils or zest in mine and i don't alway mirror the cake flavor. for example, on a banana cake, i may make a caramel symple syrup.

i only use them when i feel the cake benefits from one and not as a rule of thumb.

I was thinking I read or heard that it was to be spritzed on the cake. Thanks for clarifying.

I use a basting brush and basically brush inside and out and even between my torted layers , I also use melted Jams and Jellies the same way .