is it useful to connect the camera to an external monitor? how?

You wouldn't normally hook it up to a PC monitor that large. Many people use small, 5-10 inch battery powered LCD monitors. This gives them a larger image than the LCD on the back of the camera but still remains portable.

Example: http://www.bhphotovideo.com/bnh/controller/home?O=&sku=1090213&gclid=Cj0...

I use a 27" PC monitor at work for two reasons:

The 7" monitor I have is SD resolution only.

Pulling focus is so much easier.

In my case, I always used to suffer from soft focus when I'd pull off the camera display or the 7" monitor. Since then, I don't have that problem nearly as much. Granted, a proper production monitor with features like peaking or zebras would be far more useful, but a large PC monitor can still come in quite handy.

It's a whole lot easier to focus and often gives better exposure results on cameras that don't have histograms or peaking features (granted you need to have a calibrated monitor).

It can be very usefull.
I don't know what camera you are using, but it can be usefull in many ocassions:
- when the camera's screen is low res or lacks tools to check exposure and focus
- when the camera's screen in on a spot that makes it hard to see, because of the view angle of because the camera moves differently than you
- when someone needs to see the image as well (like the director, the makeup artist, the focus puller or the client)
- when you can't trust the colors and brightness of you camera's screen

I use it when shooting with a MoVI (the gimbal blocks the camera's screen) or a slider (not having to follow a screen on a moving camera makes such shots a lot easier).
Sometimes I have to shoot with Canon DSLRs: they lack many tools the camcorders already used to have in the 90s: using an HD monitor (although small) with exposure and sharpness tools really makes a difference.