What are all these spam subscribers doing here?

Question

We look after a bunch of WordPress sites none of which run themes which involve public participation. So basically the setting Membership - anyone can register is always turned off.

I have just taken a look at one particular site and found a few dozen mystery subscribers in the user list. Checking the settings I find this site is has anyone can register turned on.

Easy enough to delete them and fix the setting.

A couple of questions however:
1) Should I check anything else apart from deleting the new users and fixing the setting?
2) What do these (presumably spam) subscribers get out of the deal? They get no access to the site, as I understand it, and no eyeballs on any spam they might want to push.

Incidentally, this site is running an up-to-date version of WordPress.

Answer 1

I'll try to answer even though the questions are a bit generic:

1) Should I check anything else apart from deleting the new users and fixing the setting?

When deleting a user from the admin, WordPress asks you if you want to keep or delete any associated post for that user. Independently from that, it also takes care of cleaning up the database (like removing user settings etc). So no, you don't need to do anything else.

2) What do these (presumably spam) subscribers get out of the deal? They get no access to the site, as I understand it, and no eyeballs on any spam they might want to push.

Most of the time they are just "random automated attacks" to gather as much information as possible, like the installed WordPress version, checking the existence of email addresses, having the ability to post comments etc...