I’m grateful for your help.
It looks like the more direct translation would be:" To you (all) i am grateful for your help". So, " Io sono grata per il vostro aiuto" to be translated as “I am grateful for your help” would be completely wrong?
Perhaps it’s more emphatic this way, more elegant. Just another way of saying “Io sono grata per il vostro aiuto”.
I have heard (and therefore said) “ti sono grato” before.
I do not know whether the object is necessary but coming from German (“ich bin Dir dankbar”) it is the natural translation for me.