Ahoy mateys.

My brother bought this thing a few years ago along with a Behringer microphone and it has sat in storage up until about 15 minutes ago.

I hooked it up to my amp to see what exactly it would do--all I could really seem to tell was that it gave me a signal boost. I'm pretty noobish when it comes to some of this stuff, so could anyone let me know if that's pretty much all that this thing does?

The settings and options on it are as follows:

Gain dial: +26 dB to +60 dB (gave my clean channel more of a crunchy sound than anything)
Output dial: 0 to +10 dB

Then 4 different buttons:

20 dB PAD
+48 V
Phase Reverse

Can anyone let me know what the original purpose of some of these things is?

I didn't spell Ultragain correctly in the title.
20db pad: make it 20dB quieter, if you're micing something thats really loud like a kick drum or a loud guitar amp

+48v: phantom power, for if you're using a condenser microphone (in which case you should plug the mic in before you turn on the phantom power)

phase reversal: flips the signal upside down, for if you are having issues with a stereo mic setup (eg: you have a snare drum that's mic'd both from the top and the bottom, if the 2 signals are out of phase they cancel each other so you're left with a crap sound, which is remedied by changing the phase on one of them)

limiter: judging from just the name, limiting is basically really hard compression (above 10:1 ratio), basically a last defence from the signal getting to hot and going into clipping