Too strong a current on the filter intake can cause tank inhabitants to be sucked into the filter or to be caught in the filter intake slots, where they die an unpleasant, slow death. There is a very high risk for shrimp or fish offspring to be sucked in. Thus, a filter intake protection can help protect the smaller inhabitants of your tank in particular from the filter intake current.
In addition, such a protective sleeve prevents aquatic plant rests from entering the external canister filter. This filter intake protection is also called Filter Guard. It is simply slipped over standard glassware filter intakes or filter hoses (given that the dimensions fit) and protects your tank inhabitants from the dangers of the filter intake current on a permanent basis.