I dont know about gas industry, but perhaps this will shed some light.
In the drinking water industry the pig is a large foam block that is shoved in the pipe, and the weight of the fluid behind it drives it along. It is used to physically loosen the accumulated debris on the inside of the pipe. This debris concentrates behind the pig, and when the pig arrives at it's destination you remove the pig and allow the fluid (and debris) to discharge to waste for a period of time until the fluid runs clear.
Some advancements include "ice pigs" which is just crushed ice. The advantage of an ice pig is you can hook up to a fire hydrant and pump the ice in, no need to access the mainline pipe itself. An ice pig will also disintegrate on its own, no need to remove it, just allow it to flow out of a downstream hydrant, any ice left behind isn't a problem.
It's very similar in the natural gas industry. The "dumb" pigs are a piece of foam that is shoved into the pipe at the "pig launcher" and they use air pressure to push the pig through the pipe, where it is then caught in the "pig catcher". All the debris from fabrication of the pipeline is scrubbed out by the pig and removed at the pig catcher.
Nowadays, they also have "smart pigs" that have sensors to detect gouges, dents, pipe wall that is too thin, etc. as the pig moves through the pipe, so that repairs can be made before the pipeline is filled with gas.
Pigs are also used to separate products. Lots of pipelines can switch between different products and different grades of products (e.g. gasoline of different specs required for different areas, kerosene, jet fuel, diesel).
In the very long fuel pipelines a foam pig can not make the whole journey to the cargo's delivery point, so the different grades of gasoline, diesel, and blendstock are pumped sequentially back-to-back with no physical separation, resulting in many barrels of intermediate off-spec fuel that is designated transmix and segregated upon arrival.
Inside the tank terminals I had to witness and certify the pigging after each parcel loaded, when one chemical was done and the line needed to be cleared and volumes accounted for before filling with a different chemical. Among many other more or less toxic things. Measurement training was not easy in the full-scale industrial environment but you can't really lead as well in the micro-scale activities without it.
In the drinking water industry the pig is a large foam block that is shoved in the pipe, and the weight of the fluid behind it drives it along. It is used to physically loosen the accumulated debris on the inside of the pipe. This debris concentrates behind the pig, and when the pig arrives at it's destination you remove the pig and allow the fluid (and debris) to discharge to waste for a period of time until the fluid runs clear.
Some advancements include "ice pigs" which is just crushed ice. The advantage of an ice pig is you can hook up to a fire hydrant and pump the ice in, no need to access the mainline pipe itself. An ice pig will also disintegrate on its own, no need to remove it, just allow it to flow out of a downstream hydrant, any ice left behind isn't a problem.