In Alli Products, filters provide a way to modify and optimize feed items after they are imported. Filters are shared among a client in Products. Since most clients have similar data across feeds, many times it makes sense to apply the same optimizations to each feed.
In both cases (feeds or deployments) there's the concept of filter order. This is a big deal as filters that are run early impact how filters that run later see the items. In other words, if a filter prepends a value on an item's title, later filters will see that change.
Filter order is important. You can make filters that are dependent on others so take this into consideration when ordering filters.