Unlike dudley, I have always used a pre-filter sponge on almost all of my filter intakes for the past 20 years. As dudley also noted, it is important to use a large pored foam and then to clean it regularly. I squeeze mine out weekly. One of the advantages to using a pre-filter is it means I have to clean the media inside the filter less often.
For example, I clean the pre-filter on all 3 of my canisters every week. Two of those canister I clean the internal media every 6 months and the third, which is loaded 100% with 20 ppi Poret foam I clean about once every 2.5 to 3 years. Since I began using Poret foam I have been switching out the old pre-filter sponges with the 10 ppi Poret foam. Also, I do use this on tanks with sand as the substrate and it does appear to prevent sand from getting into the filter. I use the Carib Sea Torpedo Beach sand. Nut i also have sand in tanks where the pre-filters are the more commin sponge material (usually made from an AC 110 foam).
I was taught almost 20 years ago by a fish keeper who worked with huge pumps, the ones which are used in large scale applications, never to impede the intake side of a pump. He told me If I needed to slow the flow from a pump to always do so on the output side. As a result it is important to rinse pre-filter sponges/foams before they clog as that will soon restrict the intake flow.
My experience is I do more maint. work on filters when I have no pre-filters than when I use them.
Oddly enough I have one tank, a 15 gal. that is heavily planted and has white clouds plus amano and blue shrimp (Neocaridina davidi
) which is filtered by a pair of AC 100s which do not have a pre-filter sponge on the intake.
All of this just proves there are very few right ways to do anything in this hobby. What matters is that the methods any one of us choose to use work to keep our tanks and thus their inhabitants in the best conditions we can provide.
Over time I have moved more and more away from power filters to sponges- more specifically, Poret cubefilters and Mattenfilters. These do not need pre-filters.