It can be to the benefit of both. The employer doesn't have to do all the onboarding until they figure out whether it's going to work. If they employee doesn't like it they have an easy out at 6 months that doesn't harm your reputation as much as bailing out shortly after taking the job. That easy out also prevents you from staying longer for fear of establishing a job-hopper reputation. At your next interview when asked why you left -'It was a 6 month contract at the end of which they offered me a full-time position but I didn't feel it was a good fit'. That's a much better story.