are lessons necessary for skiing/snowboarding

Myself and the other Faceplant regulars have been boarding for a number of years now, and we still take lessons. Getting proper lessons will make the difference between flying around enjoying yourself off piste and in the park, and spending 3 years trying to teach yourself how to carve properly on red slopes.

A good instructor will nip your bad habits in the bud before they become ingrained, plus you'll pick up on their wealth of knowledge and experience just by riding around with them. I can tell you some stories of hairy sh1t we've done on account of our instructors that we'd never have had the balls to do otherwise! (nor the experience to have gotten away with it alive most likely, had we gone for it on our own)

Which brings me to this: apart from basic skills, bear in mind that people die every single week in ski resorts due to getting caught in avalanches and other stupid crap. You'll eventually want to explore the backcountry, but you could be taking your life into your own hands if you don't have the guidance of someone who knows what they're doing, or at very least advice that was imparted beforehand. Avalanches are hard to spot.