The answer as to why SE allows anonymous users to suggest edits is the same reason as to why registered users with 1 rep can suggest edits
Can I earn reputation?
When a suggested edit is approved, the user who suggested it gets +2
reputation. The regular daily reputation cap applies, and the total
cap for reputation gained via suggested edits is 1,000. Like any other
reputation, the +2 is reversed if the edited post is ever deleted.
When a suggested edit is rejected, no penalty is given. Once you have
earned the edit privilege, your edits are no longer peer reviewed and
no reputation is given for edits.
What about abuse or bad edits?
There are strict limits enforced. If a user (anonymous or registered) submits many rejected edits they will be automatically banned from suggesting edits for 7 days.
The fixed size queue also helps protect us from abuse.
and on the SO helpcentre which I have always presumed provides the basis for all the other Stack sites
When should I edit posts?
Any time you see a post that needs improvement and are inclined to suggest an edit, you are welcome to do so. The original author of a question or answer may always edit their own post, regardless of reputation level.
For some users suggesting edits is an easy and quick way of earning the mythical reputation points. It is one of the many ways a newcomer can ‘earn’ certain privileges.
Remove New User Restrictions
Privilege Type: Milestone Privilege
Awarded At: 10 Reputation
Privilege Type: Moderation Privilege
Awarded At: 15 Reputation
But not such a quick way to earn the right of commenting everywhere …
Privilege Type: Communication Privilege
Awarded At: 50 Reputation