Differences between matt and silk paint
Paints, in order of shininess from least to most, go:
Flat-Matte-Eggshell-Satin-Semi Gloss-Gloss-High Gloss

10+ years ago:
The reason shinier paints are shinier is because they have a greater concentration of binding agents (adhesives). Because greater amounts of adhesives provides a sturdier paint, glossier paints are more durable and washable than flatter paints.

Satin is generally considered a good middle-ground. It's flat enough to not show tiny imperfections but strong enough to endure washing.

Today:
As the chemicals in paints have evolved, flatter paints have become more durable and washable (provided you're dealing with a quality paint).  I'd wager the adhesives in matte paint today do a better job of binding the material than the adhesives you could find in a gloss paint from the 1990s.

Today, it's just a question of:
How shiny do you want your paint to be, keeping in mind that the shiner it is the more imperfections it will show?

(Personal note: I'm pretty thrilled by this development. I never liked shiny paint and just a few weeks ago painted my kitchen with Sherwin-Williams Duration matte turquoise.  Duration is one of their lines that offers washable mattes, and S-W has a very good track record to their name.)