You don't NEED anything unless it serves some purpose for your playing. I'm reasonably certain I could get by with just an OD and reverb. For that matter sometimes I go straight cord into the amp. But I like having a delay in addition to the reverb. And I like having both an OD and a Distortion each set not more than half way up. This gives me three options for additional drive: OD alone, Distortion alone, or stacked. I suppose I could accomplish that with two ODs set at different levels, or two distortions. And of course there's always the volume knob method. It all depends on what works for you in your context. What are you playing that makes you think maybe you should have an OD?