Plein air painting is conducted outdoors and ideally in one session. I plein air painted every Saturday for 15 years with classes taught by Suzanne Kelly Clark, soaking up the light and fresh air of Texas (except for July and August!) Besides enjoying the outdoors, you see subtle changes in light and shadow within three dimensions which a photo cannot capture. A photo flattens space and is pretty much just light and dark. Even if I end up finishing the painting in my studio, it is better for being created plein air, looser, fresher, etc. You really need to be in that landscape and experience the light and colors of that spot to convey that onto the canvas.
(c)Ann McCann, "Golden Moment," 11 X 14 Oil, 2017