You don't need to DPS, it isn't a requirement outside of high end progression raids. However, if you ever find that you spend a few seconds not casting any heals, those seconds could be spent DPSing instead of waiting for damage to happen. That's the basic mentality of healer DPS - there's a bit more to it, including pushing the limits of your tank's health and all that, but that's a bit more risky and takes a lot of skill/know how of your job to pull off. Again, you don't need to DPS but if you ever find that you're not spam healing the tank, throwing in some DPS won't hurt anyone.