They will put on less growth, which will reduce pruning costs and use considerably less water and fertiliser. Of course, while a garden is 'young' you may well want to have maximum growth for a few years to reach the 'mature' look quickly. The amount and frequency of watering will vary considerably between species and on different soil types. It is possible to water in such a way that the grass will stay green, while growing much slower reducing mowing time and all related costs. If your soil is water logged then the grass will have very limited roots and will needs watering much more often than if you have deep healthy roots.
Drainage is essential in heavier soils.
Only water when the grass is beginning to suffer visibly and then water with quite a lot. You should use a raingauge to measure what you are applying. Within a few days the grass will start to show signs of 'stress' - you will learn where this generally occurs and use that as the 'trigger' to start irrigating. If you then apply 5mm only, the water will not be enough to soak into the soil to any depth and most of it will evaporate from the surface very quickly, leaving the grass under 'stress'. Water goes into the soil like a 'wine glass pyramid' and therefore to re-wet the soil to 30cm will need a lot of water. .