You are right, that is called a soda keg. And they are used for homebrewing beer.
This. It's due to how commercial kegs are sealed. Commercial beer kegs use what's called a Sankey fitting on the top. A special valve is installed with a ball to keep the contents in and sealed. This is far too cumbersome and expensive to acquire the correct equipment to be able to drain, clean, and fill for the home brewer.
The soda kegs (Cornelius kegs by proper name) were produced back in the day for both Pepsi and Coke and came in two sizes and two configurations. Pepsi kegs are the more desirable as they used ball lock fittings as opposed to pin lock. The most common size is the 5 gallon ones you see pictured in this thread, however there were some 3 gallon ones produced.
The benefit of using these is they have a removable center cap. This makes it very easy to open, drain, clean, sanitize, and fill for the home brewer. The residual co2 pressure is bled off, the cap is opened, you drain out the remaining dregs of beer the tube could not pick up (usually less than half a cup worth) and you can actually get a brush down in there to properly clean it out. To fill the beer is just transferred via a hose from your brewing bucket or vessel straight in. Then reseal the cap, hook it to co2 for a day or two and the beer is carbonated and ready to drink.
In this pic of the inside of my kegerator you can clearly see the removable lids with the small pressure relief valve in the middle. The little wire cage thing just flips up and the center drops out