I noticed a new house being built yesterday with rain guttering all round the roof. Never seen guttering used here before. I always assumed it wouldn't be of any use, cos when it rains hard, the water would be too much and overload the gutters. Just another thing to maintain. Any ideas why a new house is getting guttering?

Perhaps they are collecting the water in their cistern?
I use this system to store water for our gardens. We use a lot of it.
Either way diverting the rain water away from the house should be rule of thumb. If the gutters are large enough and installed properly the water should flow with no problem. We have regular sized gutters but with 4" PVC down spouts and piping, works fine. Of course there are always the storms that downpour really hard and is nearly impossible to shed all that water at one time.

ah, that could be it!
thought there must be a logical explanation.
you say that diverting water away should be rule of thumb - but why? nobody does it here. doesn't seem to affect anyone negatively, or does it?

Gutters can eliminate excess water away from the homes exterior walls. The walls should have footings and drain tile around the footings. I have yet to see drain tile intalled here in the DR, except for my home. Also, when a new home is built the soil around the immediate building is disturbed and should be tamped or packed down properly when filled back in. This too is almost never done. Settling is eminent.
Concrete and mortar absorb moisture. The less moisture the better off your structure will be. Another preventitive measure is to tar or seal the exterior concrete and mortar that is below grade. This waterproofs the perimeter and should be done with any basement or below grade room from penitrating moisture.
Gutters also can protect the landscaping around the home from erosion, plant damage/ breakage and overwatering/ root rot.
Remember too, that when entering/ exiting the home, a gutter can protect you from a waterfall of rain.
Gutters do cost and need to be maintained.
But my personal opinion is that it is worth the extra cost and effort to maintain.

good explanation! never thought about it before.

We installed gutters at our house mainy beacuse the rain damages the wooden windows, so after that we don't have to paint the windows that much as before and it protets the wood.