Hello, Diana and Mr. Ahmed!
May I say my point of view?
As Mr. Ahmed_btm said, "value" should be uncountable after "of".
1. These benefits are of important value.
‘of’ is stating that benefits possess important value.
Your sentence could be written in other two ways:
2. These benefits have important value.
3. These benefits are valuable.
There's another one, but it has a different meaning:
4. These benefits are important values.
(1, 2 and 3 are saying that the benefits have important value, whereas 4 is saying that the benefits themselves are the important values).