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Morning dew is not enough to consider your lawn watered. Typically you want a few inches of saturated soil for watering.

As for why it's not "ideal" to run sprinklers in the middle of the day, the idea is that if you water early in the morning, then all of your sunlight hours beat on watered grass. If you just started watering in the afternoon, your grass has already been in prime morning sunlight without the watered soil to boost growing. It's not necessarily bad, it's just not ideal.

It's because the rate of evaporation is highest during the noon hours. Water utilities regularly ask people to water their lawns in the early morning or evening to reduce wasting water to evaporation before it can be soaked in by the grass itself.

A lawn doesn't need a constant mist, it needs a couple of inches of water a couple of times a week. Dew is unlikely to accumulate to that depth. Watering in the morning reduces evaporation.

If you water in the middle of the day, you'll lose more water to evaporation before it can soak into the soil.