Pressurized portafilter baskets (sometimes called dual-wall, double-bottom, or training baskets) include a small valve that creates artificial backpressure regardless of how poorly the coffee is ground or distributed. They produce decent-looking crema and a passable shot from any grind, including pre-ground supermarket coffee.
Why they exist
They are a beginner accommodation. Without one, a fresh espresso owner with a blade grinder and supermarket coffee gets a thin, watery shot with no crema, gives up on espresso, and returns the machine. With one, the same setup produces something that looks like espresso, even if it does not taste like it.
Why they are limiting
The valve creates pressure artificially, separating the act of pulling a shot from the actual variables that produce good espresso. With a pressurized basket, your grind size does not matter much. Your tamp does not matter. Your distribution does not matter. The shot looks the same regardless of what you do.
That sounds convenient until you realize you cannot improve. The machine is producing a shot that is independent of your skill. You cannot dial in. You cannot taste the difference between fresh and stale beans. You are stuck at a ceiling that is far below what the machine is actually capable of.
The upgrade path
If your machine came with a pressurized basket, the best $15-20 you can spend is a single-wall (unpressurized) basket of the right size for your machine. Most Gaggia Classics and Breville Bambinos accept aftermarket VST or IMS baskets that fit perfectly.
You will need to upgrade your grinder at the same time. Pressurized baskets exist because most beginner espresso owners also have inadequate grinders. With a single-wall basket and a real burr grinder, you start brewing actual espresso.
When pressurized makes sense
Honest answer: when you cannot or will not invest in a real grinder. Some users buy an espresso machine for occasional convenience and do not want to also buy a $150-300 grinder. For them, pressurized baskets and pre-ground coffee are a reasonable compromise.
Just know that you are not making espresso the way most enthusiasts mean the word.