3. Are they just colloquial expressions? 4. What are idioms designated for? (What are they used for?)?

Idioms are not just "colloquialisms" - indeed, most are not colloquial. The term idiom simply means the way a language says things, but we specifically refer to idioms when things we say don't necessarily necessarily conform to the pattern we expect, especially between different languages. Idioms are not "designated" - no one sits down and says: "OK, let's invent a new idiom". Language evolves and develops of its own free will and therefore can relied upon to defy logic and to be unpredicatble.