A HUGE benefit!
I bought a bike a few months ago due to having knee problems and I love it. It gets your heart rate up higher than walking will ever do.
You can make it harder by using higher gears, finding hills or cycling on a windy day
You don't have to spend a fortune on a bike either. I bought a Trek 3500 mountain bike and got thicker tyres and tubes (needed for where I ride) and it was about $400. After seeing bikes for hundres and even thousands more I asked why this one was so cheap and he said it just didn't have features that I wouldn't use anyway. Sounded good.