​​CBO is gonna be required starting this September

"In September 2019, you won't be able to turn off campaign budget optimisation for new campaigns, including campaigns that are more than one year old.

What to expect:

You'll have the ability to set one central campaign budget to optimise across ad sets, and we'll automatically and continuously distribute the budget in real-time to your top-performing ad sets. Facebook algorithm thinks they can optimize your ad sets based on real-time data, therefore you won't have any control of the budget within each ad sets.

In real time, we'll capture the most results for your budget, effectively lowering your total cost per result.
On average, campaign budget optimization generates 12% more conversions and 5.4% lower CPAs (cost per action) for eligible campaigns.

Campaign budget optimization and the lowest cost bid strategy help you achieve the lowest cost per result for your overall campaign, not for any given ad set. Learn more about campaign budget optimization reporting.

Sources: https://business.facebook.com/business/help/773289396357218