I see them in so many local fish store aquariums and in youtube videos, what do they do?
They basically provide biological filtration for the aquarium
Sponge filters offer a great amount of bacterial filtration since they are porous and have lots of space to house bacteria. My sponge is also great at picking up litter (mostly leaves from dead plants), and since there’s no internal moter there’s no fear in jamming or clogging. Sponges do need a clean now and again—just squeeze them out gently in a bucket of tank water and all the goop should be gone.
A lot of people like them for small tanks or for fish who prefer reduced water flow. I also use a sponge on my 20 on the opposite side of my HoB to give it a little kick.
Pretty much what everyone else said. If your bubbler is too strong, it will produce as much, or more current than a HOB or canister.
Absolutely. That's a good way to keep your BB stable. I ran mine for a week after putting a hob on my little tank, then for another week or two at night.
I don´t have a sponge filter, but I am planning on getting one soon to supplement my hang on back filter. This video will give you a good introduction to his kind of filter. Then you search on Youtube for more videos on sponge filters, many experienced fishkeepers have videos that really praise them...
Even if you don't need the extra filtration, it's great to have in your tank anyway. If you get new or sick fish you have an already cycled filter on hand for quarantine.