An even, readable dose

A clear solution at a known strength is what lets you read a dose as units on a U-100 syringe instead of guessing. Mix 10 mg of powder with 2 mL and you have 5 mg per mL, so 2 units is 100 mcg and 20 units is 1 mg. The arithmetic only holds if the liquid stayed clear and the vial stayed good, which is what the preservative buys.