Happy 4th Everyone!

My apologies for such a newb question, but what exactly is that black spherical thing for that's under the hood? Mine has a few dry rotted hoses running to it that I imagine should be replaced.

 

It keeps a vacuum hold on the HVAC system inside. The flapper doors in the dash that direct airflow are controlled by vacuum actuators (excluding the defrost actuator which is electronic on 98+, I believe 94-97 are still vacuum actuated). If the reservoir wasn't there or the hoses become rotted and leak so that it no longer holds vacuum, you'll start noticing when you accelerate the vents will stop blowing air or the air temp will change.

This is because the engine creates vacuum at idle or when the throttle is just barely cracked up (when cruising). When you open the throttle up more (such as when you're accelerating) the vacuum in the intake starts to go away. The HVAC system needs that constant vacuum to hold the flapper doors in their selected spots, and if the vacuum disappears, they loose their hold, which lets air flow the wrong ways or at the wrong temps. The vacuum reservoir is exactly what it's name implies, a reservoir for the vacuum when you're accelerating. It holds vacuum long enough for the HVAC system to continue functioning properly while accelerating and the engine isn't producing any vacuum.