The best feature of iTunes is the marketplace, that allows you to browse music, videos, podcasts, ect. and download them to your computer. While having an iPod will be great for synching up this media, it is not necessary to have this in order to download content through the service and use on your computer. As far as playback, iTunes does allow for a library, playlists, playback, equilization, visualization, etc. There really is no difference between playing media on Windows Media Player and iTunes - just different menus.
However, if you do want to sync these to a media player and take them with you, some version of an iPod will be required. Itunes codes their media so it is playable only with their players and devices. This is not at all inconvenient if you are using an iPod/iTunes, but can be annoying if you're trying to find a way to get it working with something else.