Minor spoilers up through the end of Season 2
The Diamonds are secretly a special type of "perfect" cross gem fusion, and protecting this secret source of power results in the Homeworld cross-Gem fusion taboo. Their rigid caste system and authoritarian politics requires the predictability that Gems behave according to their assigned function without question or ambition. The Diamonds grew their original power by assimilating exceptionally talented Gems--willingly or forcibly--into a single-Gem entity.
The tighter the bond of the fusion is, the more humanoid the resulting Gem will be. For example, a close fusion like Garnet does not posses the extra limbs or more monstrous qualities of Alexandrite or Malachite. The Diamonds are so tightly fused that they have literally transformed into new, structurally altered Gems that cannot unfuse--there is nothing to unfuse.
This perfected, stable fusion requires Rose’s unique healing abilities and is demonstrated in Steven’s creation. Her healing powers (or Steven’s spit) are shown to effect inorganic and organic material, and so she was able to fuse her Gem with Greg’s genetic material to transform herself into Steven. Her healing abilities allowed her to have a stable pregnancy, and is somewhat analogous to a Gem seed being injected into the earth to gestate. Here, the resource consumed was a part of Rose herself; this gave Steven “space” to exist as his own entity in the Rose Quartz gem.
I believe Rose (as Pink Diamond), at some point denounced her role in the conspiracy and possible forced assimilation of other Gems into the Diamonds. Her rebellion against the Homeworld may have openly begun before the invasion plans for Earth, but it’s likely that it had at least internally begun in this scenario. Part of why she loved the Earth--and then humans--was its ability to change and grow. She herself wanted to find a way to grow without cheating, lying, or subsuming other Gems. This eventually led her to “grow” into Steven as a way to finally achieve something she so wistfully desired.
Pink Diamond used her abilities to structurally transform herself into a new Gem, formally renouncing her power and status. To do this, she may have had to excise some amount of power both to alter herself and to seal it away from the Homeworld Gems. This excess power needed to be anchored somewhere, and so Pink Diamond’s Pearl may have volunteered to bear it. The resulting fusion of a part of Pink Diamond’s power with Pink Pearl resulted in the imperfect fusion that is Lion. If our Pearl is any indication, Pearls may be predisposed to loyalty and devotion to those they serve, resulting in Pink Pearl’s willingness to sacrifice herself to transform into a new--and possibly not full sentient--Gem creature. Pearl in particular has been known to store and summon items in her gem, and Peridot commented that Pearls were meant for display and to “hold stuff”--they may have doubled as very fancy, living handbags. Pink Pearl’s natural abilities were magnified when she was infused with the power that transformed her into Lion. The gateway to the pocket dimension may actually be the transformed Pink Pearl gem (though I think someone might have noticed by now. Further, the other Diamond Pearl’s we’ve seen have gems in the same place as their owners, so I would expect Lion to have a belly gem unless the transformation/corruption process altered this).
Lion is very similar to some of the corrupted Gems, particularly Centipeedle. Without her full powers, Rose could not fully assimilate Pink Pearl into a fully humanoid entity, leaving Lion as something like a benevolent version of a Gem monster. Rose kept Lion a secret in order to prevent the power from ever being fully harnessed should she or other Crystal Gems be recaptured by Homeworld. The Diamonds continued to crave her ability to create perfect fusions; Homeworld continues to experiment with forced fusion via the Cluster and other mutants to re-create the power they lost when Pink Diamond abdicated.
Eventually, I think Steven will find a way to unseal and reclaim Rose’s full powers and become a new, more compassionate Pink Diamond to finally help bridge the Earth and Gem cultures. None of this hinges on Rose’s identity as Pink Diamond being a secret, though that would add to the intrigue and explain why Lion was hidden.