There aren't many air con systems that are totally gas tight, a bit like tyres, a lot of them have very slow leaks and when you're talking years then that's a very slow leak indeed.
Even if it is gas tight (and some systems I know can last a lifetime without leaks or need topping up) the thing is, it could really do with a drain down and refill as otherwise it's the same oil going around the system year after year and that's counter productive and costly in the long run as there's a compressor in there working hard with little pistons going up and down like the clappers
So, having an annual system check, drain down and refill is well worthwhile and can make a big difference to efficiency otherwise the compressor is working twice as hard all the time