Summer jackets are perfect for hot motorcycle rides and warm weather conditions. Those days that you'd rather be riding in a T-shirt, yes, but that's not only illegal, it's also extremely unwise and unsafe.
A summer jacket often consists of (large pieces of) mesh fabric, which gives free play to the wind. This mesh is specifically designed for motorcycle use, and is wear-resistant. Moreover, a jacket also has room for all other protective gear.
Summer jackets can also consist of perforated leather; literally: leather with many holes.
Basically, for anyone who wants to enjoy some comfort during the summer and doesn't want to melt on the way south. More specifically, for those who feel hot very quickly. Summer jackets now come in all styles and models: touring, adventure, sporty, retro...
To choose is to lose. What ventilates and focuses on breezy feels can't possibly be waterproof. Nor is it windproof, to be perfectly clear.
There are summer jackets which are standard equipped with a removable water/windproof membrane, which is especially interesting on chilly mornings or during rapidly cooling evenings. Logically, you can never expect the same from a summer jacket as from a full option textile jacket with membrane for daily use in all conditions.
If you want to be surprised on the road, it's best to combine a summer jacket with a good rain coat.
The outer fabric and membranes of textile jackets are more demanding than, say, a T-shirt or a pair of socks. Everything is possible, but specific maintenance products for motorcycle clothing are really no marketing trick. General detergents can damage the properties of membranes and outer fabric, so always take a close look at the information for maintaining motorcycle clothing.