Why does France need six nuclear powered submarines?
In addition, they can be used on their own to launch strikes against high value targets (things like the chemical weapons facilities in Syria). They are also very useful in the Indo-Pacific to deter China (France still has territorial possessions in southeast Asia): https://warontherocks.com/2021/04/how-franco-australian-coop... .
You need a bunch because half of them will be docked (repairs, replenishment, R&R, etc.) at any particular time, and having just one at sea risks an adversary taking it out in a first strike.
France "needs" all this equipment because France (or least enough of its governments) have decided that it's important for France to have an independent capability to influence the world beyond "soft power".
It's easy to understand why they might want an independent SLBM force. And if you're going to be building nuclear submarines, may as well make nuclear attack subs so you can try to keep the knowledge and industry base more continuously active.
In a surface attack role a nuclear sub has a significant speed and endurance advantage. Conventional subs are can't adjust to course changes in targets very easily; of course, they are quieter and smaller, so choke point ambush becomes more feasible.
A nuclear boat also has the speed to keep up with a carrier battle group or a fast convoy. The ability to use SLCM to strike land (MdCN) or maritime targets (Exocet) without obvious forewarning is also useful.
Strategic boats are not used for conventional attack because it would reveal their location, and the launch of a ballistic missile could trigger real escalation towards nuclear exchange.