If you don’t bury your grass seeds, your seed’s germination rate will decrease because you will, inevitably, lose some of these seeds. Some people would often sprinkle grass seeds throughout the area they are trying to grow grass; unfortunately, these seeds become susceptible to the climate. For example, if it rains, your seeds could go awash; if it's too hot, seeds could no longer germinate due to heat stress. You need to help the seeds grow, and you can only do so if you offer them the right environment to thrive. Next time consider adding some mulch after planting the grass seeds and observe how different your growing experience will be.