for free = freely 免费地（多用于口语）：The old people can use the facility for free.老年人可以免费使用这设备.
It's free.这是免费的,空闲的,自由的.由此可见,free除了作免费解外,还有“空闲”、“自由”的意思.
it's for free与it's free的区别主要是用在具体的语境与上下文不同.如果仅仅就一件东西或某种服务来说它是免费的,既可以说it's for free,又可以说it's free.