Grass seeds need water to germinate (sprout from the soil) and then to continue to grow.
After sowing your seeds it is crucial to keep the seed bed moist, not waterlogged though. Having constant access to that moisture means more of your seeds will sprout.
A poor watering regime means dead seeds and bare patches so make sure at this stage you keep an eye on your soil and give them the best start for healthy growth.
1. Before germination: Less water but more often evenly give soil a good watering 2 times a day (remember if it’s really warm soil will dry out more quickly so keep those seeds hydrated. Lightly and evenly is the best way. Make sure all the seed bed has been sprayed
2. After germination: More water but less often - now you want to encourage those roots to grow so having good supply of water below the soil means giving more water but less often as this helps the roots push down into the moisture in the soil. Grass seed root systems take time to mature and if you neglect to water they will suffer
3. Watering: Use a sprinkler or hose with spray attachment. Flooding the soil creates other problems like rotting or seeds floating off with no good soil contact.