Here is the difference between shared and VPS
I agree that the price is really a benefit, but sometimes the price means nothing if the quality of the shared server on which your account is located leaves much to be desired.
When one user overloads the server, this badly influences the other customers located on the same server. You may have a simple blog and there can be a number of users running live chats and heavy scripts on the same server and thus, your blog will become heavy and slow as well.
The only solution from this situation is either moving to a more stable shred server( when the server is new) or buying your own private server ( VPS). Having your own VPS server, you will not have to share the server resources with the other users and you will pay just for what you are using.This is one of the main benefits of the VPS server over shared hosting.
If you know at once that your site will need a lot of resources, it is better to buy a VPS server at the beginning as later you will have to ask the hosting provider to move your files from the shred account to a VPS and this may badly influence the site performance and some data can be lost as a result of the transfer.