Mainly it was SAP (Special Access Programs). It's how she sold secrets to China, etc. Other than that for back channel communication with her other criminal cohorts like Obama, etc. If you are looking for a "legitimate" reason, there isn't one.

Q drop #3046

The server used to be in the basement when Bill first left the White House.

He was asked and he said he really didn't use it, Hillary did. Bill acted text stupid. He said he didn't do email or anything because you just got into trouble. Hillary used it. That's what he said in public.

We know the server was set up to run an entire banking money laundering system of Black market ops. Alongside the grifting Clinton foundation and global initiative.

From there Hillary kept using telephones and things she knew she was not supposed to use. SHE DOESN'T CARE!

Wait til you find out about obummer's private email addy

SCIF

If you’re trying to hide a server from the authorities, probably the best place to hide it would be in the bathroom.

They’re going to go into your home office and take away all your computer equipment but who is going to check the linen closet in your bathroom?

Sorry, no links handy. Spoiler- the server was a digital "dead drop" location for smuggling highly (extremely) classified data to the highest bidder. Data would be loaded into the server, then some brilliant "hacker" would "break in" to the server by cracking a password- like "password1" and "steal" the classified data. It was done that way to aid a plausible deniability defense should they get caught. Also, Colin Powell advised Hillary on how to set up the illegal server in a way to make it difficult to be detected by the law dogs.

The server was hidden in Hillary's basement, was illegal for her to even posses, and had laughable security. It contained classified, secret, top secret, and ultra-top secret S.A.P. (Special Access Program) data that Hillary did not have a security clearance for.