It's very useful for any language. There are sounds of every languages, a lot of them I don't have idea which language has them. To understand well how to make the sounds, first of all you have to study a little bit Phonetics and Phonology (You'll find a lot of pdf articles about that). Then, it'll become very easy for you to understand the places of articulation of each letter and you'll be able to reproduce those sounds.

In this site, you can see the phonetic symbology of the words and know how to pronounce them: http://www.wordreference.com/enit/witchcraft

I use it on occasion if I came across an unfamiliar word, then thats where it shines. A couple of years back, I had to find out how to pronounce the word "insipid". Without the IPA (most good dictionaries will have instructions in the first couple of pages on how to use it), I would probably still pronounce "insipid" as "inspit". 

It has its uses, and I recommend it highly.