Photo By: HVargas
For the same reason that plumbers do really. Think about it. The VMware tools are optimized set of drivers for both ‘virtual’ hardware like the video card & network adaptor, and in ESX/ESXi’s context provide some of the advanced features, like memory ballooning.
Tools it a bit more than that, though. One of the benefits of virtualization is portability – you can move a virtual machine between computers (in some cases, even as the virtual machine is still running). But what if the first and second computers have different hardware – how can the virtual machine possibly cope if it starts off on a Mac mini, gets suspended, and resumes on a Mac Pro (or maybe even a computer running Windows or Linux and using VMware Workstation, VMware Player, VMware Server, or so on).
Virtual machines don’t see the physical hardware directly (which would cause portability problems). More importantly, nearly all hardware expects to be controlled by exactly one OS – even if the guest could see the physical hardware, it would cause lots of problems. Instead, the guest sees a set of emulated hardware that doesn’t depend on the actual hardware; Fusion takes care of translating appropriately.
All hardware, even virtual hardware that doesn’t really exist, needs drivers for OSes to know how to talk to them. For some of the virtual hardware (for example sound) we emulate existing physical devices, the guest can use standard drivers provided by the OS or third parties. For other virtual hardware (for example video) we use our own "hardware" that doesn’t exist in the physical world. While it obeys a basic set of commands, it needs drivers to get all the features (such as 3D acceleration). These drivers are a part of Tools.
That there is a write-up by the fusion team on the need for the VMware tools as well. Their post is mostly geared to Fusion, but applies across all of the VMware product line. Hopefully this helps clear up what they do and why you should be installing them.