Prepositive modifiers don't like to have postpositive dependents. The more common pattern employs prepositive dependents:
It is a very easy app.
The "very" is a prepositive modifier of "easy", and the phrase "very easy" is prepositive to the "app" that it modifies.
Another common pattern has postpositive dependents for a postpositive modifier:
The app is easy to use.
When a prepositive modifier has a postpositive dependent, there is a conflict. The prepositive modifier is expected to modify the next word, but there's a postpositive dependent competing for that same word position.
The way to solve the word placement competition is to treat the phrase "easy to use" as a single word. The hyphens join the more tightly bound phrase. Inside the hyphenated phrase, the expectation of the postpostive dependent position of "to use" is fulfilled. As a result, the "I'm modifying the very next word" expectation of the prepositive "easy-to-use" is easy to fulfill.
This is an easy-to-use app.
Postpositive modification inside the hyphenation, prepositive modification outside. The shift between right-to-left parsing and left-to-right parsing is clearly marked.
There is another word ordering that might be worth comparative consideration:
This is an easy app to use.
In this example, "to use" is definitely a postpositive dependent, but the word from which it depends is ambiguous. In this position, "to use" could modify either "app" or "easy".
This ambiguity does not result in any word-ordering conflicts. Regardless of whether "to use" depends from "easy" or from "app", it is in its expected postpositive position. The boundary between left-to-right and right-to-left parsing does not need to be marked.