Knock sensors in many (probably most) cars retard the spark allowing the use of lower octane fuel but with a performance penalty. But as long as the octane level is sufficient to eliminate knocking without retarding the spark then there is no reason for additional octane.

I've got a 2000 Ranger with the 3L V6 and manual transmission that knocks on regular in the summer months and needs midgrade. But it doesn't improve gas mileage. And although I usually use "Top Tier" gas, Chevron, I found that I get better mileage using Costco gas. Back in the late 1960s I saved money by running Gulftane (a sub-regular grade gas) in my VW with no loss in gas mileage.

Sometimes premium grades have more additives, but it would appear that in the Top Tier brands all grades get the additives. Thus the only thing you are buying with "premium" gas is increased octane which you may not need.