This is a relic from ancient CMK versions 
Before you had the possibility to select the nice settings for agent and SNMP, there was only the option agent, SNMP, booth or nothing.
And only one datasource per host was allowed.
With this trick it was possible to have two datasources on one host. vCenter was also possible to install on normal Windows servers and then you wanted to have monitoring for the Windows and the vCenter.
The same applies to the first Linux vCenter appliances. This was a normal Linux with vCenter software.
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