Painting en plein air is a time honoured tradition. Most of my instructors and mentors extoll its virtues. I've done loads of workshops and courses en plein air - most recently this month - and I do always see the value. You work quickly, which often allows you to capture the mood and lighting of the scene in a way that is very fresh. It's excellent practice at simplifying complex subjects and creating compositions. It's difficult too - conditions change rapidly and the light you are painting in doesn't match the indoor lighting under which your painting will be viewed.
Here's one that I liked that I did in a recent workshop. "Esquimault Lagoon" Acrylic on canvas panel.
Of course, I should say that I hate it. It's always too hot, too cold, too windy, too buggy. It's hard to find a comfortable spot and your paint dries too rapidly. I rarely like my plein air work compared to my studio work. I usually just do it as a challenge to get me out of my comfort zone but honestly I'd probably have a better time just sitting around a campfire or reading a book on the beach if I want to spend time outdoors.
Still, I keep trying .... just joined a local plein air group and plan to start getting out there after some upcoming travel is over.