do fescue lawns not need to be watered as much if periodically spray liquid fertilizer on it?  how come?

We don't know where you are.

Many home owners are opting for 100% subterranean sprinklers which are 100% underground and you would never known when they come on or off.

This eliminates damage to sprinkler heads and eliminates evaporation/wind etc.

I use granular fertiliser on my lawn and I have no idea what sort of grass it is but it stays green without water for longer when it is fertilised. I

The ones from the trucks, on a regular schedule and such, gives the lawn all they need to get through issues and such. The problem with chemical laws, as this are often called. Is that they never develop a true stable root system, stop the chemical treatments and the grass will die.

The pellets slowly dissolve, forcing the roots to search a little more, but the same end result if one stops the treatments and such.  The chemicals spray has the advantage becouse they have to be done more frequently, which means that lawn is also getting watered more frequently. If you really want that lawn to be good throughout the season, an irrigation system is a must.  If you want a lawn that is somewhat decent, but will also stand up to some neglect, then you want a mix of grasses that look good under different conditions through the season. Use your spring, summer, fall treatments, and let it do what it it will.