Patrick - I check my body temp in the morning because I have low T3 and I'm trying to fix my thyroid (with Dr. G's help).
While some degree of variation may not be a problem for someone with normal thyroid function, I've noticed a clear correlation between my AM temp and how well I feel/function on that day in terms of fatigue, motivation, mental clarity etc. If it drops below a certain point I might as well be hibernating
Also helped me figure out what things I do may be detrimental (fasting too much/too long, drinking coffee regularly, keeping carbs contantly below 30g) and what may be helpful (carb cycling, vitamin D).