It is purely a personal feel for why we have rituals are what they are and his highly speculative.
I feel it is purely a mechanical reason behind it. To prevent certain games/scenes to become ludicrous, there was a need for some spells to be impossible to cast rapidly, repeatedly, without cost.
Specifically: healing spells.
When Ars magica 2nd ed came (I cannot talk for 1st ed), the standard for medieal RpG was to be some form of heroic adventure. If a magus can cast repeatedly healing spells, you remove the heroic part of most challenges. Hence the invention of ritual: it allows to recover from a almost deadly adventure so the flow can resume, yet, it does not make a combat lasting forever since nobody can "instant heal" in the heat of the action.
This is what a ritual does: it allows access to some spell that would mechanically break the adventure if they were cast without limit (and therefore should not exist).
How could magi be unpopular if they could heal any wounds, any illness with a few seconds of casting ? You can slam as many malus you want on social roll, when somebody is on his death bed, they will let the healer reach them. And they will remember it. For them, for their wife, children, parents. After a few people saved from their deathbed by a magus, no matter how smelly and foul his presence is, they will learn to appreciate him and trust him.
Other games are/were limiting this potential abuse with number of use per day, or fatigue, or any other economic (meaning there was a trade off with a limited currency of some form). Ars was one of the first game saying: if you are good at a spell, there is no reason you should not be able to do it all day long, even for powerfull spell.
But there is the need to have a way to control certain spell in a elegant way, with some form of logic/mechanic: rituals were invented.
Now, there is a nice background story explaining why Creo healing spell should require virtus. But you can go through 2nd ed, and there was nothing like that. It was just: healing spells are ritual full stop.
This concept of ritual was nice and convenient, and was applied to other spells.
With the overhaul than 5th ed brought, because there is nice, systematic rules, this kind of issue are creeping, legacy of the past.
So there is a need and room for rituals, for the sake of balance and game mechanics.
Yet, if we apply raw, it creates some interesting statistical issues. It can be handled, but it need to be capture in the background of the Order or a change in the rule:
Case 1: the botch rule remain the same, but mastery allow to reduce the risk to 0 if cast in relax circumstance: every covenant has a "Master of the Aegis", who is in charge of casting the ritual
Case 2: the botch rule remain the same, but mastery only reduce by 1 the number of botch dice per mastery level: there are specialists that can be hired for a few pawns to cast the ritual. They are probably mercurian magicians with the virtue allowing them to apply their mastery to similar spell (forgot the name). It means that for a few days around winter solstice, some covenants won't have an Aegis, until the specialist finally arrives. Or will have one, but cast by somebody without the full safety net. Politics and favors are traded to make sure that the specialist is available on time for those who can afford it. Much opportunity for stories.
Case 3. Aegis is an oddity which does not conform fully to hermetic theory, therefore .