why every startup and every big company uses #wordpress ? what is the benefit of using it ? fast management , easy to use , customization instead of creating one with html , css , js and back-end (php) ?
The big advantage of using an existing CMS (content management system, like Wordpress) compared to developing your own is that you don't have to invest the effort to develop your own. What's the point in doing that? Wordpress already exists. Startups especially don't have a lot of time and money to spare; they need that to develop the thing they're actually planning to make money with.
Wordpress in particular is time-proven, flexible thanks to many available plugins, and extensible in that if some functionality is missing, you can write your own plugin with much less effort than creating something from scratch.
As a general rule, unless it's about your core competency (i.e. the thing you specifically want to make money with), try to use something existing instead of developing it yourself. Unless your company specifically wants to make CMSs or complex web sites that go beyond the capability of common CMSs, don't write one.