Here are some more examples for the usefulness of radar trails. All of the photos have the trails in true mode, in other words, the actual track of the target.

This first example is coming in to the eastern side of the straights of Gibraltar. The targets without trails are not moving (in this case ships at anchor).

The trails here show a bunch of targets moving out from shore. Given that it is 0400, the odds are these are fishing boats.

Here is a close up of the previous photo. The consistent course and trail length indicate these targets are not yet working with gear in the water.