As Michael Dale notes in the foundation's announcement, "One potential problem with increased video usage on the Wikimedia sites is that video is many times more costly to distribute than text and images that make up Wikipedia articles today. Eventually bandwidth costs could saturate the foundation budget or leave less resources for other projects and programs. For this reason it is important to start exploring and experimenting with future content distribution platforms and partnerships."
If you visit Wikipedia without SwarmPlayer, video is streamed from the normal server. But with SwarmPlayer installed, the .torrent is requested and downloaded from the peer-to-peer network. To ensure smooth video playback, a fallback mechanism to the normal Wikipedia video servers is implemented.
SwarmPlayer can be configured to determine how much bandwidth you use to upload. And even if you upload nothing, using the add-on allows you to help distribute video loads by playing it from the P2P network - and helps ease the load on Wikipedia's servers.