A) how did spectating start here in the first place?
That's a really good question. I have played a few games that didn't have that kind of thing, but it was such a very long time ago. I've seen far more benefits to specs than downsides.
B) what specific roles do mods see for the spectators?
They help keep players engaged and motivated for the game. Also, having extra eyes is super helpful. I rely on spectators because often things can be missed if I'm the only person reading things. They've helped to point out mistakes in results or challenge posts. They are good to point out when there are situations going on which may need a closer eye. These games are huge and as much as you try to read everything, it's not completely possible to do that. And having active spectators makes that be less of a problem. They're also a resource for the mods because the mods have someone they can talk to about what is going on with whom and what they're excited about and can't wait to see what players do. And they can get feedback on their moderation from a less biased perspective. It motivates the mods to keep going as long as specs are being particularly good that game anyways. There is the downside of sometimes specs can be particularly toxic and make it so that mods have a harder time wanting to keep modding their games when there's no appreciation or positive feedback given.
C) what roles have spectators believe they have filled in games before?
I know I've judged challenges as a spectator, and a few times I've stepped up to help run challenges that needed a mod to start. But I've played the part of being extra eyes for mods. And I've done the motivation cheerleader role for both players and mods. Just having a voice telling you that they are excited to see what you're doing and cheering for you can make everything better. Can make the particularly bad game days that much better. Because these games are often very taxing and just having kind voices makes giving up less likely.
D) as a player, how likely would you post your thoughts without seeing posts from spectators?
This is my biggest problem. If I don't have active spectator/mod interaction, I can't be assed to confessionalize. There's just way too much to get out there without some sort of direction of what to put down. And spectator questions and posts have always helped. My confessionals end up being a lot less updated or well put together without specs. Having banter with specs is also very motivating and enjoyable. Sometimes you may not get to questions, but having them is a huge help. Helps to organize, which for me anyways, is a huge problem. I'm extremely scatterbrained and when there's just so much it's not something I can just pick one thing and go with, I need the starting point. Questions that aren't vague but ask about specific things help so much too. Having an incentive to post, ala Hogwarts/LoL also is helpful for posting, but without specs, it feels hollow and like talking to a wall. Feedback is so vital.