The base Pelagos is essentially Tudor’s current generation Deep Diver model that also includes standard dive depth variants (39, FXD, RedBull x Alinghi).
It has no backstory really other than it is derived from various back catalog models from Rolex/Tudor shared heritage. At the time it was constructed mostly of titanium excluding parts of the bracelet. The quick adjust system included an interesting innovative spring based quick expansion system intended for quickly accommodating a dry suit, or standard neoprene dive suit. It is rated to 500 meters and has a helium escape valve.
While the Submariner is the standard dive depth model from Rolex. The Rolex equivalent to the Pelagos would be the Sea Dweller.
When Tudor first launched the Pelagos in 2012 after making a comeback in the US market, the marketing angle was (still is for the Pelagos) towards the working man’s tool watch narrative reminiscent of Rolex past. Their ads portrayed divers, explorers, and drivers wearing Pelagos, Black Bays, North Flags, and Heritage Chronos.
Nowadays, that narrative doesn’t sell anymore so the marketing for most of the models has predominantly shifted towards the socialite and popular youth sports.