These "bugs" as they are called do actually serve a couple of useful purposes.

First of all, there are normal cable systems out there with upwards of around 200 channels. As the Big Four networks have noticed, broadcast television ratings have been falling pretty rapidly compared to the ratings they were getting back in the 60s-80s. This is because all of those cable channels are robbing them of viewers. Think about how much television you watch, and think about what portion of it is on a major broadcast network like NBC, CBS, ABC, or Fox. Now, how much of it is one of those other 196 channels? If you said you watch more cable than the major networks, you aren't alone. The major networks are trying everything they can to make themselves and their shows well known because they are facing pretty large falls in profits. Those little icons in the corner are part of branding logo recognition, and making sure that people know what channel they are looking at. Also note that most TV sets nowadays either automatically detect or let you program what number is what channel (for example, the little TNT that appears in the corner when you hit "Display" on your remote). This is yet another way to make sure the viewer knows what he or she is watching (although less obtrusively than bugs, since they aren't hard coded into the signal...)

The second reason for these icons ties into the competition between channels. Before bugs were common, many channels would steal other channel's video and rebroadcast it. I don't mean entire shows, but things like exclusive video on the news, things of that nature. If another channel chooses to broadcast that video and it has the original channel's logo, they give their competitor free advertising and tell the whole world they are stealing. However, this has become less important as networks usually just cover up the other channel's bugs with their own so the viewer doesn't see the logo on the original video, just the thieving channel's icon.