I first did it for practical reasons, then I started doing it as a style.
Leggins under any kind of trousers are very comfortable and diplomatic.
Men usually have long trousers and do not show legs. Why should women show naked legs during daily activities? On the job, women could wear leggins in place of trousers. Much more comfortable.
Leggins under knee or short trousers protect from sweat, from insects bites, from showing water retention, from showing unshaved skin, from bushes in the forest, from anything.
Leggins under shorts still show silhouette and allow women to feel sexy.
They also allow men to show their body. Leggins are not only for runners. They are a very unisex/bisex.
Men have beautiful legs, too. Why should they show it only in the gym or on the beach?
Attention must be paid to choose the right stuff. Too much elastane under the sun gets warm. But elastane gets dried quickly and is very comfortable against wind and during the night as night clothing. Finally, runners leggins are very resistant to scratch.
During summer time, you can wear wet leggins and stay under the sun. You will feel refreshed and safe from hot hits. They will dry quickly.
Moreover, if you use some soothing tonic water, you will also hydrate all your body as a face mask or hydrating gloves for hands. Your skin will be kept soft and protected from scars or calluses.
Finally, with leggins under shorts you can go everywhere in the world without cultural worries.
Simply choose the right one for the time you wear them.