No point if produces max torque/power below the red line
actually, there is.
lets ignore torque (its irrelevant at this point) and focus on power. A typical power curve might go
4.0 150bhp
4.5 160bhp
5.0 170bhp
5.5 180bhp
6.0 170bhp
(redline)
lets say your shift from 2nd to 3rd drops from 6000 rpm to 4,500 rpm – this would be going from 170 bhp down to 160bhp. if however you shifted at peak power (5500 rpm to 4000 rpm), you’d drop from 180bhp down to 150bhp (ish, it would actually drop slightly fewer bhp as the revs would only drop to something like 4125 rpm). anyway, from the time it takes you to get back to 4,500 rpm you’d be getting ~150-160 bhp. if you’d let the car rev, you’d have been getting ~180-170 bhp at the same time (using the higher revs), which is a lot more.
On very few engines you do get less than maximum power out of the engine by shifting early (ie ones with a very peaky/early power curve), but on the vast majority of engines (yes, even including turbo diseasals), you get most power by hitting the redline every time. That’s why the autobox does exactly that.