There are a few reasons why you should buy a VPS but not a shared Web Hosting.
- if you want a full root shell access.
- update a kernel fixes at your own timing (only possible with Kernel independent VPS like KVM)
- hosts a few websites and full control with better management.
- install the components or applications.
- allocate unused resources.
- restart services at your own preferences.
- with a better budget.
- better isolation.
- dedicated IP address, it is optional in Shared Hosting.