We know that it's part of the Wizard's Sight.
We know it lets a wizard see the core or essence of a person (and be seen, in return).
Several seem to be "snapshot of the soul" events, where some archetypal symbolic "tableau" is emplaced. Thomas in the black-and-white desolate Olympus, facing his demon in the mirror; Marcone's sterile steel "tiger's soul;" Rasmussen chained to the cliffside; etc.
Others are different.
Helen Beckitt was re-living her daughter's "death," a clear and distinct memory.
Molly was a hall-of-mirrors many; like a bunch of future-soulgazes (which launches a whole new notion -- are we seeing the first instances of Harry having time powers alongside his other known strengths?).
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Maybe it's no more than Jim using them for dramatic purpose; that there isn't so much a "unified theory" as a vague theme centered around what can happen in a Soulgaze.
Is there WoJ? Or a more-expansive exposition scene by Harry, or Bob?
(I post here in Spoilers in case there's a Soulgaze I don't know of, in a recent short / comic I haven't read)