Not an expert, but I’ve seen 3 Distinct timings for Dynamaxing: when you want to suddenly sweep with a set-up Sweeper (Having Flying/Fighting/Poison moves help boost stats further), when you want to temporarily change which move a Choice Item Holder uses (to allow more efficient sweeping), or when you want to counter a Dynamax (usually using Max Guard at least once), Granted you think you can KO the opponent as they revert. :P