Around big hills, the 'turn back' option is never as easy as it sounds.
If you can't operate higher than the terrain due oxygen, aircraft performance or whatever, your option is to fly through the area using gaps and valleys.
If cloud is forming around those areas, or vis is bad in heavy rain etc, you can find yourself forced close to the ground, crawling from clear space to clear space and possibly getting trapped in a valley if it clags in at both ends.
There may be no landing site (thick trees etc). The valley you're in may get too narrow to easily turn in. You may not have the performance to hover if you're heavy and high.
Worse, if you inadvertently enter cloud close to the terrain (easy enough to do) you must either get visual again quickly (descent is likely to fly you straight into the ground), or navigate a safe course out in IMC (climbing or continuing level in cloud will probably fly you into the hillside).
There are ways to keep yourself safe (always have an escape route behind you, don't press into a narrow valley that you can't turn in etc etc), but it's only too easy to get squeezed into some predicament and have to work your way out of it. If you didn't launch in bad weather in some places, you'd never get anything done, so a compromise between the pressure to get a job done and the need to fly safely develops.
With IFR capable aeroplanes on air routes, the decision-making process is generally clear cut and relatively straightforward. For the helicopter crew in bad weather around big hills, it's a constantly evolving battle against conditions and circumstance.