What does an artist do when painting a landscape "en plein air"?

When painting "en plein air" the artist is painting outdoors in the fresh air on location. One of the challenges historically with painting outside is the mixing of the paints often is a challenge and difficult to bring on location. The obvious advantage of painting landscapes on sight is the more accurate portrayal of the actual landscape. When this form started photographs did not exist.

The theory of 'En plein air' painting is credited to Pierre-Henri de Valenciennes (1750–1819), first expounded in a treatise entitled Reflections and Advice to a Student on Painting, Particularly on Landscape (1800), where he developed the concept of landscape portraiture by which the artist paints directly onto canvas in situ within the landscape.

https://en.wikipedia.org/wiki/En_plein_air