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 the wind free play. The chosen mesh is specifically designed for motorcycle use, and is wear-resistant. Moreover, a jacket also allows for all other impact protectors.
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 find it too hot very quickly. Today, jackets come in all styles and different models: touring, adventure, sporty, retro... and women's models boast an adapted cut, are centred and even quite fashionable.
To choose is to lose. What is optimally ventilated can't possibly be waterproof. Nor windproof, just to be clear.
There are windproof 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 don't want to be surprised on the road, it's best to combine a summer jacket with a good raincoat.
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 look carefully at the information for maintaining motorcycle clothing.