A) Personally i started spectating when I was cut from a player list. It's just a thing that happens here. I'm yet to play any offsite, but I believe all the ones I've located have something similar, whether they have posting access in confessionals or not (as I haven't looked at them harder than that). For in general, probably just someone asking if they could watch after a game filled, that or it was based on the same type of information that dead threads in mafia do, and how those sometimes attract additional spectators.
B) Keep the players engaged with finding out what is happening in the game, modding without information such as that just means we don't get as good games as the mods aren't able to deliver to the same quality of service.
They're also helpful in being able to occasionally help run challenges, and can also playtest things beforehand, and have an unbiased opinion without generally knowing who is behind each of the players. (although this lately isn't true)
C) I think I just answered this with my answer for B, probably because until I run a game with an additional use for them, the above answers are what i see as a spectator that generally hasn't actually done most of that stuff.
D) Based on how much I have posted in my confessionals between each of my games, I think how much I post in my confessional has actually become due to what I've seen in other people's confessionals after games have ended. I don't believe having the spectators there has directly help me post more than I otherwise would have, but it is hard to tell.
Like my first survivor (Mario Bros), I would probably have just avoided a confessional if I was actually given one. (PTs didn't exist yet on site, so it would have been a quicktopic), This is probably because I basically never actually messaged many people privately, and instead was just using tribe chats. After learning about that information about the need to message for my second game (Doctor Who), I think I utilized my confessional with the help of the spectators. Even just a couple of posts right at the start of the confessional (even if it's in the Welcome thread) probably help everyone get to using them.
I consider any confessional I have my personal blog, and will use them even if we took spectators away from access to posting in it.