I find that a refeed day helps me with my training. My body fat has reduced enough, and if I don’t do any sort of refeed I find that my muscles get weak and I start to lose energy - the opposite of what keto normally does. Once a month or so I will have a carb-filled day, and my stomach always hurts a little and I get foggy, so I know I’m not at risk of slipping back into bad eating habits. Carbs help my muscles, but every other part of me feels gross. If you have any extra body fat you probably don’t need to refeed, but if you’re doing long endurance training I find that it helps. Exogenous ketones also help me feel better when I have more carbs than usual. If you’re at risk of falling off the wagon entirely, then I imagine refeed days are unnecessary, but if you start to feel lots of muscle weakness with endurance training, you might want to try.