No one needs write access, but most systems only have a read/write predefined role for tenant-wide access. If you don’t trust the department staff to give you anything but a predefined role, it’s typically the only option. Similarly if you need to fire privileged IT staff on the spot for headcount reduction you need admin-equivalent rights to lock them out. You can’t in general trust disgruntled admins to lock themselves out!

Also, in some cloud systems full read access can give you direct or indirect access to service keys / API keys which then are write equivalent permissions anyway.