The Pontiac Transmission Filter used in the automatic transmission is not a very common filter to be changed out. It is not because it is not important; it is only because most people are actually unaware of its existence.
Unlike the engine oil pan that has a plug in it that an owner can use to change their own engine oil, the transmission filter is behind a pan with no plug on your Pontiac. This gives most owners the impression that the oil inside of it does not have to be periodically changed out. This is a false conception but is prevalent by most owners.
The changing of the engine oil filter is done ten times more frequent than that of the Pontiac transmission filter or about every 30,000 miles. The process of changing the transmission filter requires the Pontiac transmission pan to be dropped. This is how the old transition fluid is drained from the system since there is no plug to make this process easy.
The reason the transmission filter and fluid have to be changed periodically is because upon being used, the oil become saturated with participates. It can also become burnt when the transmission is overtaxed when it is being used. This generally occurs when a heavy load is being hauled or the Pontiac has to drive up steep hills on a regular basis. The transmission filter can filter out most particulates but not the ones that has burnt transition fluid in it. This requires the fluid to be changed or the transmission will not be able to shift properly. The Pontiac owner can identify this when they smell the transmission fluid and it has a burnt odor or it.