Polypropylene is a cheaper alternative to Acetylene. There are differing opinions on this, and under the circumstances may not be in the best taste to debate it today, but many folks believe Acetylene is, in general, safer.
At shops like this, the gas is used as a flammable carrier medium to apply various (mainly powder) forms of high alloy metal to steel. The main tank was probably piped into working areas to be used in it's various applications.
HVOF is one method used to coat steel products, but Watson had several other coating methods that required significantly more polypropylene.