Policy describes intent
A policy can say that Alice and Bob sign, or that a recovery key may sign after a delay. Compilation selects a concrete Script form. Different scripts may express related intent with different cost and privacy trade-offs.
Types enable analysis
Miniscript fragments carry type and property rules that let tools check stack behavior, malleability conditions, resource limits and maximum satisfaction weight before funds are received.
Descriptors provide wallet context
A descriptor wraps the expression in sh(), wsh() or tr(), adds keys and derivations and can include a checksum. It therefore connects a spending policy to reproducible address derivation and recovery.
Tools do not choose risk
Analysis can show that a branch is satisfiable and estimate its cost, but cannot decide whether keys are independent, a timeout is sensible or an heir can follow the procedure. BIP 379's status should also be checked by implementers rather than treated as a consensus rule.