There are times when you cannot log all of your meals / activity in a day - this data skews averages / aggregated statistics in the my data tab. By either default excluding days with no meal entries (would need to add an option for fasting), or by adding an option to exclude days from statistics (could do this at the app level "exclude all days with no entries" or at the individual day level "exclude today"), you enable more fault-tolerant (and therefore useful) statistics.

Example: much of apple health works this way. e.g., if you log caffeine consumption, and then aggregate the 6 month view, it does not count any days without a log towards the averages (ie it does not assume "0" for those days)