In a nutshell, besides physical, local access - there is no more of a risk of not having a password than having one. Besides, the obvious risk of anyone being able to walk up to your computer and log in.
There are a number of risks however that come from fraud and fraud prevention. Since anyone can walk up to any computer and log in, if anything malicious were to happen (and it does) you would have no way to prove who it was.
EDIT: if you're on windows 7 you can use autologin: https://technet.microsoft.com/en-us/library/ee872306.aspx
FRAUD AND FRAUD PREVENTION
Typically, an organization would have controls in place to prevent fraud from occurring in the first place. The idea being that the more difficult it is to prevent fraud, the more unlikely it will be that it will occur. I'll outline a rough scenario at the end if that helps clear it up for you.
A common control method is proper passwords, so that someone couldn't walk up to a computer, use it for some malicious activity, and then walk away. Keep in mind that this person may be from outside the company as well - ie. could be a repairman, could be an auditor, health inspector, etc. depending on your line of work, not just someone from inside the company.
An organization has no passwords on their computers - An employee, Jane, was one of three employees let go due to work force reduction from a budget cut. Her job was to manage the company account for making purchases from new suppliers.
A few weeks after she was let go, her replacement noticed some irregularities in the accounts that she was in charge of. An internal investigation was launched, which found that almost $5000 had gone missing over the past 18 months. An external auditor was called in to assess the situation and a fraud investigator acquired them to find out what happened.
They discovered that the Janes computer was used to move the money, however it was always during her out of office hours. They also discovered that no-one in the company uses passwords on any of their computers, meaning that it could be anyone who would've had physical access during those 18 months. They also found from conducting an interview that Jane left with no issues, and left them with little reason to think it was her.
So who could it be?
Same as above except employees use passwords -
They discover that Janes computer was used to move the money, and it was always during out of office hours. However it was not her login details that had been used, but rather the hiring manager for the floor.
An investigation was launched and found that he was having some issues at home with his spouse, which could be a potential motivator.
The point being that in each of these situations, even with passwords being used its difficult to say who it was that committed the crime. However, in the case of scenario 2 - the hiring manager would have to explain why he logged in with his details outside of normal hours, or how his credentials had been potentially 'hacked'.
Hope that clears things up a bit. I can expand further if not.
EDIT: Just to add since @Sayan's point - it is known in security as non-repudiation - or being able to prove that something happened by whom