Linux is customizable in a way that Windows is not. For example, NASLite is a version of Linux that runs off a single floppy disk and converts an old computer into a file server.
Linux can also run from a CD. To run Windows, it has to first be installed to your hard disk but there are quite a few versions of Linux that run completely from a CD without having to be installed to a hard disk. Windows can not run from a CD.
Installing Linux on a computer without an OS is much easier than
installing it on a machine with an existing OS that you want to preserve.
Linux is very cheap or free, Windows is expensive for home PC and on the other hand for server use, Linux is very cheap compared to Windows.