It does not absorb water, meaning any H2O in the system will cause corrosion/mechanical issues. Water in the system would vapourize and cause the vapour lock issues This implies that you need to flush it often, but race folks would do this anyway. You get higher boiling points, with this caveat.
For street use where fluid is left in for a few years (or more), you would not want Castor SRF, but rather a typical DOT 3, 4, 5.1 which absorb moisture and prevent rust.
Because any water introduced to the brake system will tend to puddle at a low point, like in a wheel cylinder or a caliper. Then, when brakes get hot this water can boil at a relatively low temperature. Steam is highly compressible, so instead of applying the brakes, the hydraulic action of the master cylinder gets used to compress the steam instead of applying the brake pads or shoes.
If brake fluid did not absorb water, the bottoms of mountain passes would have way more brake failure types of accidents.
Not getting any air out of the lines so it’s not quite that hands off. You still need to bleed all corners periodically.
I’ve owned this tester for 4 years now and it’s been great. I live in a dry climate and brake fluid for me lasts ~4 years before it hits 2%. In reality in a dry climate doing half the work every 4 years and having a test prove it is worth the peace of mind.