I will go with
I am a tool but I do not really exist
I am doubtful about the term existence here. Languages exist, but they have no physical existence.
I am always used, even when I am not
Even if you don't use a specific language, you still use a language. Or even your body language!
The people that truly benefit from me are those that have trouble using me
When you go to a country who speaks a foreign language, and you have trouble speaking that language, you benefit from understanding more than most of the people because you need to understand. Also, as pointed out by Sam Hazleton, the flexibility of language (it is only a protocol for interpretation after all) can greatly benefit those who are impaired in a given form of communication (e.g. braille, sign language)
I am found everywhere, but I haven't always been the same
Language indeed is found everywhere, but it is different for each country, and even each region in a country.
I am never the same, but I am always constant
English, for instance, evolved, improved, but we still call it English.
Everyone understands me, even those that don't know me
Babies, dogs, cats can't speak, but they can somehow understand you when you speak.