Many medium-large corporations have their own cloud environments that their IT Ops control. Solution providers can host Atlassian stacks on their own cloud environment where they are not affected by data privacy concerns (it's in their already green-lit cloud providers data center) so they can host it behind a firewall with only VPN access allowed. They can also do all the magic you can usually do with web software like put a frontend proxy in front of it, or use more flexible/legacy authentication methods. Not to mention that for example you could have a Jira Cloud that you would need to integrate with a SCM program. Jira data could be "OK" to live in the cloud but code would be a big no-no. These problems can be solved by having them all live behind the firewall.
A competent managed solution provider also has consultants that can train or instruct on usage. It costs but it is simpler and faster than having to go through the forums or send a support ticket for every small issue to Atlassian itself.
If it's some multi-tenant solution it's no better.