I was reading that for business's it is essential that they remove the user's account as soon as that person leaves the company. So, my question is why? 

I understand the need to remove accounts which can accessed remotely, as the person who left the job could be disgruntled and leak the data to a competitor etc but I can't see the point of removing a user account on a computer immediately.

I know this question sounds really simple but I can't think of a specific reason why. 

Any help would be greatly appreciated, thanks.