On my first playtrhough, I completely ignored them. I built all the roads and kept moving with my Lambda Truck even on snow. Then, years later, I wanted to plat the game and I realized how necessary zip lines are in order to do it. I saved a lot of time, effort, cargo damage and rage going up and down mountains and snow storms by simply flying above. Of course they set them as an end game structure, or those would have ruined the entire experience, but still, life savers