Brake fluid is hygroscopic so yes it will draw moisture from the atmosphere. When it does that reduces the boiling point of the fluid, if the brakes get hot enough to boil the fluid it turns to gas, hydraulic systems don't work well with vapour, you run out of brakes.

I think generally every 2 years for a fluid flush / change but they probably test the fluid for moisture content. Also as moisture increases that too can cause corrosion in the brake system.

Brake fluid is cheap, it's a good preventative to do a change every now and then

 

Every year I strip down the wheels and the brakes also bleed them too. Rust belt is **** on disc brakes.

 

I change the brake fluid every 20 years just as the manual says. Even if it doesn't need it.
If you drive in a mountainous area I would change it more often but I've never had a problem with just following the manual's recommendation. In this case the manual says leave it alone.

 

Don't know what service manuals you guys read but mine says every 2 years.

 

Mine has no mention of brake fluid service ever. Maybe yours is a UK or Canadian extreme condition thing?

 

Brake fluid is like antifreeze. Its lucky if it ever gets changed by anyone. Every two years sounds a bit anal to me.

 

Well I suppose in Canada it lasts longer with lower humidity levels. I test every car I service and replace the fluid when it gets over 2% moisture content which can be as soon as a year or up to four years but if left it will vaporise on a hot day and the owner complains of a very spongy pedal that fixed itself the next day.