Top dressing serves 3 main goals: to reduce thatch levels, smooth a mowing surface, or improve germination and establishment of a seeded area. The selection of the material varies depending on the goal associated with your turf. Typically, a mixture of compost and sand are used, like the mix in the video below. This service is usually accompanied with an aerification to cultivate the material into the top 2-3" of soil and is then dragged to break up the cores and move the material into the turf evenly.
For seeding jobs, this can improve distribution of the seeds, reducing the polka dot appearance at germination, typically seen when a core aerification is done prior to seeding. However, please keep in mind that Tall Fescue has a bunch type growth pattern, regardless of how it is planted, so if it is not periodically over-seeded it will eventually begin to look clumpy or choppy.