This video shows a graphical comparison between stage1 tune and stage1 dyno tune on AMG GT, done by Eurocharged Boston. and both graphs compared to stock.

Disclaimer: There is no one size fits all; some cases a dyno is a MUST. particularly stage3 and heavily modified vehicles with large turbos and fuel pumps etc.
One the opposite side of the spectrum, some entry level cars with basic or no mods will always see no gain that justifies dyno tuning.
Most cars are somewhere in the middle. Always talk to your tuner that you trust to best advise you on how to approach your particular project!