As an anecdote, I can say water-only fasting in combination with a high nutrient density plant-based diet has helped me overcome some serious allergies and other health conditions. As an added bonus, I've lost 65 pounds from my peak weight over the last few years and my weight has been stable. It's not a panacea, but I've found it to be a net benefit.
For those interested in a deeper dive on the benefits of water-only fasting and diets that try to mimic its benefits, as well as time-restricted feeding, see:
Valter Longo, Ph.D. on Fasting-Mimicking Diet & Fasting for Longevity, Cancer & Multiple Sclerosis [1]
Dr. Valter Longo on Resetting Autoimmunity and Rejuvenating Systems with Prolonged Fasting & the FMD [2]
Dr. Satchin Panda on Time-Restricted Feeding and Its Effects on Obesity, Muscle Mass & Heart Health [3]
Dr. Satchin Panda on Practical Implementation of Time-Restricted Eating & Shift Work Strategies [4]
[1] https://www.youtube.com/watch?v=d6PyyatqJSE
[2] https://www.youtube.com/watch?v=evGFWRXEzz8
[3] https://www.youtube.com/watch?v=-R-eqJDQ2nU
[4] https://www.youtube.com/watch?v=iywhaz5z0qs
[Edit to fix typos and formatting]