I'm going to chime in because I'm the one who pushed these Charms and one of them was a recycle from one cut out of Abyssals. Holden was gracious enough to include them.
In the case of Abyssal access, it serves a variety of functions. First, it allows you to have the Anakin Meets the Sand People effect where the Solar delivers Hungry Tiger Technique for the first three actions of his flurry and finishes off with artful maiming onslaught and all his Circle-mates are looking at him like "Damn!"
It also completes a symmetry, that when you turn Solar Essence inside out, you get an Abyssal. The ramifications of that are terrifying, wherever you go with it. Or perhaps this isn't quite the case, but instead an Abyssal is what happens when Solars delve too hard into necromancy on their long fall down. It's not remotely beyond the pale to imagine a followup Charm that you can buy that lets you change yourself into an Abyssal completely on your own.
The Charm also incidentally lets you do a First Age "Abyssals" game or include still living pre-Deathlords with hints of what they will become.
As for the primordial Charm, the prerequisites are the first explanation. Sorcery is a pattern that the Primordials made for themselves and which they were the greatest wielders of right up until Mara said "Here, catch!" to Brigid. Primordial Charms are very often sorcery-like, if not outright sorcerous, and certainly do a variety of things Solar Charms simply can't do for matters of theme (rather than power). For a solar sorcerer who wants to press harder and deeper into this power, the path to cosmic power (Primordial Charms) beckons past Essence 5. It's a sideways leap because Solars are at once the most human of Creation's Exalted and yet have achieved all of the most intense transhuman feats in the setting. It's a paradox, but it's a cool paradox.
We also know by way of the existence of Green Sun Princes that Solar Exaltation is strong enough to power Infernal Charms and can do so. An Infernal has nearly infinite sideways direction to grow because each Charm set they unlock will be radically different. Plus, because multiple sets can plug each other's Charm holes, diversity is the Green Sun Prince weapon for fighting with the reliable perfection of a Solar. However, if you tack on Solar Charms to plug the holes as a development option for any Infernal, then this short-circuits the omni-yozi evolution push. Thus, the Infernal set functions best as intended without this kind of broad access.
In contrast, Solars don't have that kind of flexibility within their own Charms. They can go up and punch right through any ceiling, but they can't innately push their Charms in an alien path. For them to go sideways, they need to do something other than be Solars. Making the Charm Essence 6 ensures that by the time you get it, you already have your core competencies fleshed out and what you're looking for is exotic tricks to flavor you, basically new "spells" to add to your repertoire as a sorcerer. But they are expensive and you have elder Solar Charms you could buy instead for cheaper, so once again, the Solar is encouraged to stay a Solar, but given the opportunity to be something…else.
Oh, and bang for your buck, those two Charms open up more options for Solars than any other Charms they have, so in a book with limited space, this was a plus.
The final reason I have for those two Charms is more personal and it stems from the fact that Infernal Charms are studies in abuse and various responses to abuse. When I was a child, if I screwed up and I knew it and I was sent to my room because I was in trouble, I would hit myself and bang myself against the wall. It wasn't an attention plea. It was me deciding that punishing myself worse than my parents would punish me gave me the power. It was me doing this to myself. Not someone else doing something to me. It was a tiny crum of self-empowerment at a time when I didn't feel that I had any (through no fault of my parents, bless them). What I also understand now is that it was a form of self-abuse. Even as I was protagonizing myself in my story, I was also ensuring that story was a tragedy.
These "damnation" Charms take that principle to a Solar level. "I will not be dragged into the darkness by some old ghost or an Abyssal; I'll drag MYSELF into the darkness and fuck you all." Also: "I will not be your akuma slave; I will steal your secrets (that incidentally carry weaknesses that make me more like you and spread your concepts, but no matter)." Even at their most degenerate fraying of every physical law they can get their shiny hands on, Solars should have the capacity to forge ahead in the direction they please. If that means they want to avoid having a Deathlord throw them into the Well of Oblivion by taking a swan dive of their own volition, that is still very much within the Solar prerogative. Solars are the captains of their own destiny. That also means they can get it wrong. Or weird. Or maybe perversely right. Who can say? All I know is that Solars are just as capable of self-abuse as spouse abuse and these two Charms capture that idea better than anything else I could think of.