I understood that ships only need rigid hulled lifeBOATS for a proportion of those on board, but must have inflatable lifeRAFTS for the rest.
The total spaces in boats and rafts has to be at least twice the capacity of the ship, in order that all on board may be saved if fire or capsize prevents the use of the rafts or boats on one side of the ship.

I believe the reason for this is twofold.

Firstly a rigid hulled boat is preferable for the sick, injured, very old, or very young, but the able bodied should be fine in rafts.
Secondly, an inflatable life raft is effectively a single use item. But a lifeboat can be launched, recovered and replaced in its davits for re-use. This can be very useful in an emergency that does not require abondoning ship. For example a sick or injured person can be taken ashore in a ships lifeboat, or a doctor fetched in the lifeboat, if the ship is too large to dock.

Lifeboats are almost allways mechanicly propelled and may therefore move against the current to land, liferafts drift and normally rely on the occupants being rescued by other craft.