Yes, downspout strainers prevent debris from entering the top opening of the downspout. However, they can cause debris to build up against the strainer and will need to be regularly checked.  Downspout strainers are similar to gutter guards in the sense that they are designed to keep out leaves and debris. The difference is a gutter guard is a long flat mesh placed over the cutter, while a strainer is a cork-shaped webbed structure used to plug the hole in the gutter that connects to the downspout.