Agree that it is essential, particularly for medical and evacuation: you couldn't even begin to guess what can happen to people overseas.
However, once you get beyond the general holiday overseas and start wanting to cover specialist trips that include activities such as climbing, canyoning, scuba diving or whatever, pay particular attention to exclusions.
Most travel insurers will exclude climbing (and I'm a climber too actually) and some or many other adventure activities, so pay attention and if in doubt ask.
They'll also have exclusions around 'deliberately putting your life in danger' (which is pretty vague I know) but means for example, climbing over barriers that have signs saying not to and then having an accident probably won't be covered.
Best advice I ever heard was: if you act like you aren't insured, would you still do or behave like that? Because if the answer is no, then your travel insurance might well not cover it either.
regards