What is a "mesh" fabric?
Mesh fabric is thinner, lighter, and more airy than cotton or polyester fabric. This breathable fabric wicks away perspiration more quickly. The material doesn't leave a damp feeling on your skin which give you chills while cycling. So the mountain bike gloves are more comfortable to wear (second skin effect).
Why wear gloves in summer?
Cycling can sometimes be a bit uncomfortable. You can feel the bumps in the road or trail through your handlebars.
And if you fall, your hands will often get scraped. Wearing the right kind of gloves is therefore a real help if you want to be comfortable and safe as you ride.