Zones are used to split DNS data into zones of responsibility. I.e. when you want different administrative entities to have control over different sets of names, you put the names in different zones, served by different name servers controlled by those different administrative entities. If a company would split, say, the HQ and research department into separate subdomains "hq.example.com" and "research.example.com", and if they made those two subdomains into separate zones, each department could have their own name servers and each department’s name server administration team could add and delete server host names in their own zone all day long without risking breaking anything for the other department, and without even involving anyone else.
More practically, the "com" and "net" zones are managed by a different company than, say, the zone for "ibm.com", therefore they are in different zones. Also, the root zone (".") and all the country-specific zones like "se" or "de" are certainly managed by different entities (governments), therefore the root zone is separate from those individual country-specific zones.
