As far as I've seen, most SE members here are active enough and keep editing stuff that needs to be edited. Are anonymous edits useful at all?
No, anonymous edits are not useful.
If someone can't be bothered to register an account (with anonymity! they can be user137103820382), they are unlikely to care about the well-being of the site. Moreover, they are almost certainly unaware of the site's culture and policies.
Most every anonymous edit I've seen has been vandalism. And the few that were good-intentioned were things that should be rejected as too minor, or things that too drastically deviated from the author's intention.
This kind of noise is detrimental to the site. Especially because poor edits do leak through the review. We have a number of users who auto-approve every edit they review, and my own experiences tell me careful reviewers are outnumbered by the careless on the site (I've had to reverse a number of harmful edits to my own posts -- edits that were approved so fast I never even saw them pending.)
Yes, anonymous edits are useful. There are two reasons.
If you are SE user on another SE subsite, but not on Physics.SE, your edits will show up as anonymous (as far as I know). I, for example, sometimes read interesting questions on subsites on which I do not have an account. If I see questions or answer where multiple issues can be improved (language, style, etc), then I will suggest an edit to improve the question/answer.
Some people will end up on Physics.SE via search engines. They do not have an account, but may be very familiar with the field. If they spot mistakes in a post, they should not be discouraged to correct them.
Of course, examples as you show are just vandalism, but in my experience they are not so common. It is one of the reasons we do have the review queue, so that edits do not show up immediately.