There are tons of videos on Youtube where they show how to level grass, where most if not all have sand mixed in.
Why is sand mixed in? Couldn't I just use the top dressing as is?
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The sand is there to help prevent the soil from clumping; it also makes it easier for the material to work its way between the blades of grass and onto the soil. Frankly, any well-screened material will work, especially if you're only spot-filling (I've never used sand but have used screened/broken up soil with very good results).
You may see people recommending that you mow the grass as short as you can before levelling it. This is to make the job easier, but note that you should not remove more than a third of the blades at a time (to not stress the grass), so you may want to mow the lawn shorter over several weeks rather than in a single mowing.
One other point - the lawn and the levelling material should both be completely dry when you add the material to the lawn.
Additionally, there are different types of sand (playground, mason/sharp, kiln-dried), and I've seen posts recommending playground instead of mason's, and other posts recommending mason's over playground, but I personally think it doesn't much matter if mixed with soil. I've also seen posts recommending straight sand over a sand/soil mixture. You may also find this thread on The Lawn Forum interesting because you'll see a lot of different opinions on this issue. Not a lot of science to back up any of the opinions, however.