Why do we need cross platform for web

I am a web developer. I am thinking about why we need cross platform for web development?

For me, cross platform for computer applications, games is clearly necessary cause we can run the applications, games on multiple computing platforms (Windows, Mac OS, Linux,...). But I have no idea why we need cross platform for web. We can just deploy our website in IIS and it will be accessible on all operating systems with a browser :)