Even when you have decided you would go with free formats and applications, you can end up owning a device that only plays restricted multimedia formats (and do not wish to spend more money until its time to get a another one reading free and open-source formats), want to watch a DVD you’ve rented or bought, some videos on the intatubes, use some Microsoft fonts to share work documents with colleagues etc. The world is not perfect :)
For legal reasons, Ubuntu is not shipped with the packages that will allow you to use the restricted formats. In most parts of the world, you are however entitled to play and watch CDs or DVDs you own (you have paid for using the license, right?) on your Ubuntu system. It is illegal to use the packages to crack the protections and redistribute the files.