I think OTA are a very important time to get back to the basics. During the season it's usually more about the big picture, and yes if there is a problem position coaches normally try to fix the basic stuff as well, but that's while juggling the weekly game plan, studying up on the next team and I'm sure at least 10 more things.
OTA is for time for them to solely focus on the basics and focus on helping the players with out haven't to worry about other stuff.
And notice, with out OTAs NT Howard Green got a LOT more out of shape and basically was a non-factor after helping during the Super Bowl run.
With out OTAs Shields clearly took a step backwards last year compared to his rookie year.
And as the NFL as a whole, tackling was horrible last year, which OTAs and no-contract practices I'm sure had part to do with it.