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 but not waterlogged. Having constant access to that moisture means more of your seeds will sprout and establish well.
A poor watering regime can mean poor results, saturated / dead seeds and bare patches / uneven coverage in your lawn, so, in the first six weeks after sowing you should make sure to keep an eye on your seeds and give them the best start for healthy growth.
1. Before germination: Water deeply and daily for the first six weeks of after sowing - in periods of prolonged hot weather or drought, aim to water twice a day.
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.