reason for hyphenating adjectives

Hi,

Is the primary reason for hyphenating adjectival phrases to have them considered as a unit, where each element is to be considered a part of the other like this? Do the examples follow the reasoning stated previously?

1. After getting a first-class honors degree ...

2. This is a what-if situation sentence.

3. ... a hard-bitten private investigator ...

4. You learn best when you have hands-on experience. -- I think this is more of an idiomatic phrase than anything else.