Do you need liability insurance if you operate a company that sells services not products?
Yes--you need to be insured for completed operations. Namely, that your service will not hurt anyone (i.e. contractors, etc.)
You will need a general liability policy to be covered and you may also want to secure an errors and omissions policy. Any company that provides a service to someone for a fee can be sued for any error or omission they cause. Many of the error and omission claims are not paid out, but in the event you are sued the errors and omissions policy will cover your defense costs which are often times more expensive then the actual claim.