Virtualization can make use of certain high level CPU features, such as VT-x or AMD-v, and as a normal user you do not have the required privileges to be able to access those functions and control the processor.
That is where the Virtualbox service and kernel modules come in, they provide a trusted interface for the user software to work through in order to access the virtualization functions. That interface has root (administrator) rights and so can control the processor as necessary.
It may well be that if you disable these services that you will loose the hardware assisted virtualization and instead fall back to a slower method.
As to what VT-x and AMD-v offer, the main features are to accelerate features that are required for virtualization:
A number of key data structures used by a processor need to be shadowed. Because most operating systems use paged virtual memory, and granting the guest OS direct access to the MMU would mean loss of control by the virtualization manager, some of the work of the x86 MMU needs to be duplicated in software for the guest OS using a technique known as shadow page tables. This involves denying the guest OS any access to the actual page table entries by trapping access attempts and emulating them instead in software.
With VT-x these page tables can be handled in a much faster manner in hardware, but the Virtualbox software needs to have administrator (or "root") rights in order to access those functions and so has to use an intermediate service.