"Optical Targeting Correction" has a Range Dial added to the Attack Data Tool. It's there to add a range up to 6200yds found from the use of the OTC Rradar, or when measuring with the scopes Telemeter lens divisions. Both Radar, and the periscopes/TBT lens divisions can measure range accurately with OTC. Just remember, before "sending" the estimated range, set the scopes compass dial to the targets estimated relative bearing. Both are "sent" at the same time when you mouse click the "send range" button on the Attack Data Tool.
Here's an image of the OTC periscope station showing the Range Dial:
The image shows the scope is retractred. The lighted retracted scope background allows you to see the scopes "compass dial". This gives you the advantage of setting both range and relative bearing (two of the four parts of the manual targeting solution) while submerged, or with the scope down. If you additionally "send" the target's estimated speed and AoB, the Position Keeper/TDC can be turned "on" keeping track of the target without needing to surface. Although I'd like to double check my solution after a bit, before committing to firing.
The other reason OTC is important when using manual targeting is with the use of the stadimeter. The added corrections to the periscope Field of View, and the corrections to the target ships height dimension make the stadimeter much more accurate. Yes, there are still plenty of ways to miss getting a "nut's on" range finding, but that's the reality of true life. No finding of the four parts of a firing solution were ever absolute. If they were, we wouldn't find the game so addictive.