One big reason NOT to set up a private email server? It's, at the very least, "extremely careless." There is not the required protection for classified information and it can be used to circumvent FOIA.
Clinton herself acknowledges this was a mistake.
Really? I can think of several:
1) If she was trying to hide the emails, well, it's her husband's computer therefore it wasn't searched as part of the original investigation. This allowed her to say she had turned over all her devices. This only came up because Anthony Weiner is an idiot.
2)Along those lines, maybe she holds on to the juicy stuff just in case she ever needs to use it.
3)Maybe she wasn't storing them herself but her husband downloaded them into his computer as a)his own little insurance policy, b)to keep tabs on his wife, c)to keep in the loop with political stuff.
4)Maybe she used his computer once in awhile to check on work stuff and forgot.
Well, we do know that investigators have determined that they could be connected to the Clinton investigation. This indicates that they saw something (metadata?) that lead them to that conclusion. Other than that, could be lunch meat, could be peaches. I guess we'll find out.