Here's another reason,
A couple weeks ago I gave this Seneca fuel prices from
Airnav on his route ahead. Where he was going was
$1.00 more a gallon than others I told him about, dont
know where he eventually went. I'm sure somehow
someone on here will tell me I can get in trouble for that.

Other reasons, from my experience on this side of the mic:
General bullshi**ing.
Talking about and comparing equipment, usually avionics.
Heads up on weather.
Terrain alerts, a big issue here out west, for the uninformed.
Other traffic, of course.
In case they need an IFR clearance in a pinch.
Airspace changes on the fly, MOA's, TFR's, etc.
....and did I say just bullshi**ing in general? Which I do more
than my supervisors would prefer.
But check this out, once I actually got an atta-boy for my, well,
non-approved phraseology. This guy comes over on freq, and
you can spot em a mile away when they have that shaky voice.
I usually try to put em at ease. Well, this guy was shaky for a reason,
he was having serious engine trouble and losing altitude in
a mountainous area. The FAA likes to pull tapes and listen to the
time leading up to an incident, which they did here, and heard
my conversational type of ATC. At first they frowned on it,
but the effect it had on relaxing this guy and making flying the
airplane the number one job, gave us a positive outcome. Panic,
especially in a pretty bad looking situation like this one, only
makes matters worse. He didn't panic and got it to a runway.

So in short, you're paying for it (ATC), might as well use it.
Chris