I was first attracted to this token because i didn't know what a pony keg was. I have an excuse though, I'm not a beer drinker. :)
Anyway, I had to find out and Mr. Google obliged me with the information that a pony keg is a quarter barrel of beer. That's about 8 gallons or nearly 30 liters. While not much for an accomplished beer drinker to consume, i doubt that I've had that much in my entire lifetime, and I'm...well, let's just say not young.
I would imagine that this token was issued to only serious customers as it's a lot to give away. If this Danville Beer is a brewery or distributor then it would likely be given to an important buyer. By the way, the issuer was located in Danville, Illinois. The token, although undated, is from 1907. The reverse is blank.
The quantity involved and the token being numbered lead me to believe that it wasn't a giveaway, but rather a device for the brewers to let the customer pay in advance safely at a time cash was still king. The customer would order a quantity of beer and get a token upon payment. The cart driver would deliver the beer and take back the tokens received on delivery. That way the driver wouldn't handle money: he couldn't jump ship with it and it would make no sense to rob him.
It was an identifier for sure almost like a tool check. The number may have matched a number punched on the keg for control purposes.