1) It is very pretty and traditionally very expensive, which seems to make things fashionable. (Though as a scientist I don't know anything about fashion.)
2) Why silk is nice to ware in both warm and cool weather is due to its chemical or molecular structure. Silk is a natural fiber (like cotton, linen, and wool) and comes from the cocoon of a mulberry silkworm which turns into a moth. Silk is made by several other insects including ants and spiders. What makes silk is different from other fibers is its length. If you look at your tee shirt or a wool sweater you will see that it is slightly fuzzy. This is because the tread that is knitted or woven to make the cloth is spun from many short fibers. For example, wool comes from the hair of sheep. Each hair on the sheep is not long enough, or thick enough to make the yarn for your sweater, so many hairs (each one is one wool fiber) are twisted together to make one long piece of yarn. Silk is special in that one silk fiber can be one mile (5280 feet) long!, because the silk worms whole cocoon is one long fiber. These silk fibers can then be woven into a smooth cloth which is not fuzzy and can be made into a fabric with a very tight weave (each thread is very close to the one next to it). Very tightly woven silk fabrics are used in cold weather clothing such as silk long underwear because the very close threads trap body heat next to your skin and help keep you warm. In summer, silk clothes are nice to ware because the silk helps to absorb sweat from your skin and help it evaporate keeping you dry and cool.
Silk is also a protein like your hair, but it is special because the molecules stack in sheets like a long flat ribbon. These sheets stack together to form to make long triangular tubes. These triangular tubes act like prisms and reflect the light that hits them in many directions and make the silk shiny.I hope this helps!