What effect does learning a spell have for a sorcerer?

I’ve read and reread Learn a Spell several times, and I just don’t understand it.

For a wizard, it makes sense – the wizard adds the spell to their spellbook. But for both of the other options, I don’t understand what effect it has.

From the feat:

If you have a spellbook, Learning a Spell lets you add the spell to your spellbook; if you prepare spells from a list, it’s added to your list; if you have a spell repertoire, you can select it when you add or swap spells.

if you prepare spells from a list

What classes does this apply to? Cleric? Surely if a character prepares spells from a list, then that character has access to the entire tradition’s spell list – what benefit does learning a spell have for a class like Cleric or Druid?

if you have a spell repertoire, you can select it when you add or swap spells.

For a class like Sorcerer, as I understand it their repertoire can only ever contain as many spells as the number of spell slots they have.

So let’s say at level 1, a draconic sorcerer uses the Learn a Spell activity to learn Fear, a spell in the Arcane tradition. They don’t have Fear in their repertoire already, and they succeed at the check. If I’m right, then can’t use that spell until they level up, or spend downtime swapping another spell in their repertoire out for that spell.

But this doesn’t make sense to me either – the way the spell repertoire feature is phrased, it makes it sound like the sorcerer would be able to swap out one of the spells in their repertoire for any spell of the same level in the Arcane tradition’s spell list (Fear included) anyway when they level up – so what was the point of learning it?

Is there a subset of tradition spells that a sorcerer knows, which is somehow distinct from the sorcerer’s repertoire?

Or is the activity purely intended to allow spontaneous casters to add uncommon-or-rarer spells to their repertoire?

If someone can explain this to me, I’d really appreciate it!