does a Section 31 need to exist?
I think its more a question of what good are Section 31 for?
ENT – Archer and crew handled the Xindi threat, renegade Augments, the Vulcan-Andorian conflict, a Romulan plot, and the TCW stuff without them. Meanwhile, Section 31 had their hand in the Klingon Augment virus, and years earlier didn’t even make sure Klaang was dead to ensure a smooth first contact with the Klingons, as that could have gone very wrong in T’Pol was not added as a Vulcan representative to the crew. And it seems like Section 31 also buried the Borg file for 200 years, allowing for the Battle of Wolf 359 to happen.
DIS – Seem unaware of the Sarcophagus ship, and a war breaks out between the Federation and the Klingons. Then the following year, their A.I. goes rogue.
SNW – unaware of a plot to resume the Federation-Klingon war.
TOS – “Day of the Dove” and “The Enterprise Incident” stand out most. Along with handling Khan and Project Genesis in the TOS movies.
S31 –
plan to do a heist for a mirror universe weapon that they don’t know is a mirror universe weapon, then get talked out of it to do a different plan, then they lose the weapon, and it’s unclear if the weapon when it exploded left the mirror universe in ruins (thereby killing and ruining billion, possibly trillions of lives) or just destroyed the conduit between both universes.
TNG – The events of "Unification”, “Chain of Command”, “Face of the Enemy”, "Descent", "Gambit" “The Pegasus” and “Lower Decks” were all handled by the Ent-D crew.
DS9 – Sisko poisons a planet to stop Eddington and the Maquis from continuing to use biogenic weapons on Cardassians, and later Garak blows up Senator Vreenak to get the Romulans into the Dominion War.
VOY – Janeway stops a Species 8472 invasion, handles the Omega molecule, and then travels back in time to stop the Borg and get Voyager home early.
LD – Cerritos pulls off a black ops on their own in "Veritas", and Mariner performed some of her own with a Klingon friend years prior.
PIC – a Tal Shiar operative rose to the height echelons of Starfleet Security, who also happened to be the architect of having synths attack Mars and kill 85K people (and the billions not evacuated from Romulus), and ushering a synth ban that killed many more. Even though Romulan showed a penchant of installing spies in Federation ranks decades prior in TNG (“Data’s Day”).
Even in the Kelvinverse, after being recruited by Section 31, Khan goes rogue and kills two Starfleet admirals (Pike and Marcus), a bunch of Klingons, and - for a brief moment - Kirk. Section 31 also almost starts a war with the Klingon Empire.
Protecting Chancellor L’Rell (even though there's supposed to be a Klingon Honor Guard to do just that), faking the death of Ash Tyler and his unborn child, installing an operative on the Continuing Committee on Romulus to keep the Federation Alliance going, and
sending William Boimler to find the source of the quantum fissures are the peak of Section 31's work. The rest of the time, they have been useless or make things worse.