A good special issue will provide a snapshot of the current state-of-the-art on a given topic. As such, the editor gains:
- An opportunity to influence how people think about the topic (by defining the scope of the special issue, and by soliciting contributions that align with the editor's goals; perhaps also by the opportunity to write an editorial/introduction to the issue).
- Exposure. The special issue gives the editor a reason to interact and engage with anyone they consider interesting/important.
- A long-lived association of the editor's name with the topic: people will continue to read (papers from) the special issue for years, whereas a workshop/conference is forgotten as soon as it ends.
- CV points - as covered in other answers.
Interestingly, some of these benefits are probably lessened now that journals are largely read online only. When special issues existed as printed books, interested researchers would get hold of a copy and at least skim-read most of the papers, and it would probably be given to new PhD students looking to get up-to-speed on the field. Nowadays it's just more pdfs on the journal website, and most readers probably won't even notice that a particular paper came from a special issue.
Finally, as noted by @AndreasBlass, none of these benefits are worth anything if the special issue is not in a well-regarded venue.
Edited to add: Many of the `classic' special issues I can think of exist in a wider context: often they arise out of a meeting or workshop. Usually the editors of the special issue also had a substantial role in organising this meeting, and the editorial task was probably a modest addition to their workload that ensured the highlights of the meeting were preserved for posterity. This is a rather different circumstance from the spammy 'We invite you to propose a special issue' emails we all get every day.