Most people feel worst around day 2/3 when transitioning from glycogen to ketosis, after that you feel great.
Why should you fast if you eat healthy and have no extra weight? Two reasons:
1) What most people think is healthy is killing them. Even if someone eats paleo or keto, which are much closer to our ideal diet than anything else, they will still do things like eat a corn-fed steak. Corn-fed meat and fish is low in omega-3 and high in omega-6 and both of these things are very bad for you (omega-3 deficiency is the root dietary cause of depression as just one example). Yet, people think "I'm eating healthy".
2) Metabolically challenged cells are a definition of cancer cells. When doctors think your cancer may be metastatic they give you a PET scan. A PET scan measures the emission of positrons out of some radioactive glucose you're given. The glucose is disproportionately taken up by cancer cells for a couple of reasons, one is that they can't process ketones. Then they show you cool pictures of where the positron-emitting glucose is in your body, which is where the tumors are. If you fast, your glucose is low and those cells don't survive since there is no glucose. See 'Cancer as a Metabolic Disease' by Seyfried.
[warning: The above is not medical orthodoxy, but then medical orthodoxy hasn't made any progress on cancer in ~60 years]