Why do Pugs have flat faces and lots of wrinkles?

Why? because a long time ago someone decided that Pug faces should
be as flat as possible. The wrinkles are there because the Chinese
emperors wanted lucky symbols on the dogs foreheads, and wrinkles were
the best way to get them.

and then Pug need special for care for face?

Care of wrinkles in Pugs varies, as some dogs are prone to
holding moisture in the wrinkles, while others do not. In general,
wrinkle care is as simply as using a tissue (don’t use a cotton swab
as you can poke too hard with it) and wiping out the wrinkles on a
weekly basis. Pugs, while having no-noses, are some of the nosiest
dogs out there, and tend to gather a lot of grungy stuff (that’s the
technical term for it) in their wrinkles. If the wrinkles also trap
moisture in them, then the moisture and grungy stuff combine and the
wrinkles can quickly be infected.