One of the main reasons overseeding isn’t successful is because the seed doesn’t have enough contact with the soil like it would do when starting a lawn from scratch. Topdressing the lawn before seeding or mixing the seed with topdressing (saves time) will help to make sure that the seed has enough contact with the soil for germination. 

The benefits of top dressing don’t stop there, however:

Thatch prevention: Topdressing will cover the thatch in your lawn and allow the grass to re-root and become denser.

Drainage: Using a top dressing with a high percentage of sand will improve your soil composition for better drainage. If you have a heavy clay soil you can also topdress with compost to break down the soil over time. If you do have drainage issues you will want to make sure you aerate regularly.

Levelling: You can also use top dressing to raise areas of your lawn so that they are level before seeding.