If you don't heat your home for a while, and it's cold enough, the pipes can freeze, break, and flood your home. It's happened to more than a couple of people I know and they're in the UK - so it doesn't need to get very cold for it to happen.
This, plus I don't want to return to a completely cold house that takes about three days to heat up to 20 degrees Celsius again. The building is built of bricks, it has a certain thermal mass and the floor heating isn't very powerful. Letting the inner temperature to drop close to zero would mean a very uncomfortable period after return.
I was never absent for more than 5 days anyway, so...
In addition to other replies you want to keep the relative humidity <60% otherwise you get condensation (your 20C air, 40% relative humidity gets cooled to 15C and that becomes 55% relative humidity, cool it to 10C and it becomes 75%). Condensation can cause mold, short circuits and all kinds of other expensive problems.
Getting humidity under 50 per cent in a freshly built house over the winter ... turned out just impossible.
We fought hard to keep it at least under 60 per cent, as you say. If it froze outside, windows in the bedroom would be covered with water droplets in the morning.
(Actually, it froze last night, quite late in the season, and the windows were wet again.)
Yeah it's a bit counter productive - you build a house with great insulation so that there is almost no heat escaping outside. And then you have to install ventilation that literally creates vacuum in your house to suck cold dry (absolutely speaking) air in to keep the humidity in check. And then you sit in the living room wondering why is there such a draft. At least new houses are built with heat recovery to solve this
Keeping humidity low is all about making sure the air doesn't cool too much, or if it does, replacing it with drier (colder) air from outside. There is a certain amount of water in air and when it cools this water will condense. Good insulation (keeping inner temp constant) is key.
Letting it get too cold could lead to plumbing damage if you parts of your house get below freezing. 15C is probably higher than I'd leave it if it were vacant, but you do want to leave the heat on and a decent buffer to account for temperature variations in other parts of the house.
I discussed the temperature with the guy who oversaw the construction and he told me that, given that the house is still pretty fresh and moist (it was finished in November '22), lower temperatures would mean a risk of mould developing. 15-16 is the sweet spot according to him.
In addition to other replies, in hot and humid climates, you still want the A/C running for de-humidification purposes. So you set it high instead of turning it off completely.
