No, it's specifically to increase the speed of state retrieval. One of the uses for that is state resolution, but it could equally well just be calling the /messages API or any other point you need to know historical state. But what do I know :)
State groups aren't really a thing for messages in a timeline, there are many easier ways of doing it, for example, simply storing the message sequentially (impossible in matrix though, due to the convoluted tree structure it uses)
But when it comes to state (where state groups are actually needed) who actually needs a snapshot of the state at literally every point in history? Any other messaging app just needs to know the current state and maybe also an audit log of the change history for audit log purposes.
In any sane messaging app (e.g. discord, slack, telegram etc.) there is exactly zero relevance in knowing the member list, room configuration, permissions and room title at exactly 2024-06-19T15:23:45Z. Who the heck cares??? Yet the design of matrix somehow makes this an integral part of every single operation.
It works like this: their server gets a request to read messages, and checks if the client currently has permission to read messages. If so, it reads the messages. If not, it returns a permission error. If you don't have permission, it doesn't attempt to analyze history to figure out whether you had permission in the past.
In fact, Discord is pretty explicit about not letting you have any interaction with things you don't have permission to view. It doesn't even tell you you don't have permission. It just silently removes them from your menu, and if you try to access them with a link, it tells you "unknown error".