I might be wrong, often am, but I would guess Bodog is a default site only because it is available to US players.
With a new PC, you have the luxury of seeking out your own faves. You can always remove the sites (something we all tell ourselves we will do, but seldom do it). If you find a site that looks remarkably like another site, you will be dealing with a network. For instance, Doyle's Room recently moved to the Cake network, where it shares tables with Cake, Players Only, SportsBook, and probably a few others. Anytime you sit at a table on a network, you will be sitting with players from other sites on the network. Important to understand this. Many players, myself included, have dissimilar names at different sites on the same network. While each of those sites will prevent you from being logged in to the same network via more than one client at a time, some players might leave and come back to a table via a different site, differnet name just to bust your balls.
Cereus (Absolute and UltimateBet) is another network where via either 'doorway', you end up on the same table.
There are others, these are just the ones I know. I believe Party has a network, but being from the U.S. I haven't seen party for a few years now. I miss Party....