If the baseline is a 30% chance of dying of heart disease and it told you that you had a 70% chance, what would you do differently? (but it won't.) Probably you should already be doing it because 30% is a big number!

That line of argument is flawed because actions have costs.

To give a simple example (and discussing whether it's precisely correct is besides the point), you can take baby aspirin to reduce the risk of heart attacks. Aspirin is a blood thinner, it makes clots (which cause heart attacks) less likely. However decreasing coagulation is not an unmitigated blessing. If you get internal bleeding -- e.g. a blood vessel ruptured in your brain -- that aspirin you've been taking could make things significantly worse. It's a trade-off.

Given this, you want to know on which side (basically, heart attacks vs. strokes) is your personal risk the highest. It is actionable knowledge.

an 80% chance of breast cancer, which is really not that different from a 20% chance

Looks very different to me -- you are quite cavalier with a fourfold difference in odds...