Bit late to this topic but it is an interesting subject. I work a lot with WordPress and hoping to do more ProcessWire.
I'd suggest the maintenance covers a client's general web needs. For example, content support, troubleshooting, on-going improvements such as new features, performance monitoring, site reviews etc. Updates can still happen and I'm sure they'd want re-assurance this is covered, but it isn't always the top priority like it is with WordPress.
I'm sure clients would rather have their developer work on improvements than installing updates.
Even if a client does not need that, it will free up more time for new builds. With WordPress I do so much maintenance I have less capacity for new projects.