How to use booleans in blender? and why are they so useful!

You may be one of those who do not use booleans because they do not know how helpful they can help in the modeling process, or you don't know how to use them, this is the right place to discover how to use them in the easiest ways! 

The model you see above, was not possible for me to make without using booleans, i as a begginer always tried to make complex models out of a single mesh, except for the separated parts of course, tried extruding and all of that stuff, but it just didn't work, until i gave booleans a try, i was able to make this model, which is so detailed in my opinion, i didn't left any detail in term of modeling, you may notice that it's not smooth, we'll talk about this later on.
Booleans are built in modifiers but they take a little bit of time to use, not if you do it once, but when you have a long modeling process, you'll need an addon, which is also built in blender new versions, if you do not have it you can download it just by looking for bool tool in google, you can activate it by going to file> user preferences 

in the left panel, a new tab will pop up which help you do different boolean operations, all you have to do is intersect two shapes, select one of them and then the one you wanna substract or add to, and click one of the buttons in the left panel, difference or union or whatever, experiment with that and you'll know how helpful it is.
something to be aware of, is that booleans are pretty destructive, they give you a clean model, but if you try to subdivide smooth it, it just won't work as expected, or you may get it to work with some hard work, you can use the remesh modifier but it is not practical, as it makes the modal so dense and make blender becomes irresponsive.