Monitoring glucose and ketones can help you develop metabolic flexibility and let you grok how easily your body cuts over to ketones. Knowing your personal realtime glucose and ketone levels helps you understand just how much carbs will kick you out of ketosis or keep you in it. Glucose monitoring will help you avoid large crashes that can lead to low energy. Among other things!
My understanding is that essentially all high-level endurance athletes are just maximizing their simple sugar intake at all times (during exercise) and knowing blood glucose/ketone levels wouldn't change anything about that intake, nor would it have any impact on crashes.
All high level endurance athletes are on high carb diets all the time and are never anywhere near ketosis (unless something has gone very wrong)
Not all. Some runners play around with doing fasted long runs to enhance fat burning adaptations. The jury is still out on if it is advisable to add to a training plan. Some utlra runners have taken to extremes.