Why External Monitoring?

Where can I learn about WHY I would need to do external monitoring?

In other words, why do I need to buy a box (like a Matrox or Blackmagic or AJA) and an external monitor?

I don't get why I need to do that.

I mean, why can't I just add another Radeon video card and use another computer monitor?

I've heard that some monitors (computer ones) are "great" to monitor with.

Now, consider I don't know how to "calibrate" either (That's my next learning step) but can't I just calibrate any computer monitor or TV or HD monitor or blahblahblah monitor and be fine?

Is it just for "Broadcast" issues and quality that I need to use an external monitor? Are there different outputs that need different monitoring adjustments?

What if I'm going to film?

I would like specific answers but if you can just point me to some good sites/pages to learn more about this, that would be great!

Thanks,
Eric