Aquarium Fish Tank Double Biochemical Sponge Filter - Black

How exactly does this thing work? Is that instead of a regular hang-on-the-side filter?

So im planning to use this and replace an old corner filter in a 5 gallon tank. Would this be worth it?

Where can you buy replacement suction cups for this filter? The suctions cups that came with it stopped sticking to the glass after 8 months of use.

Is 4 of them enough for 30 gal or so?