How do you want your 7 & 8 hole hitters to approach their AB's early in games. Aggressive or working the count?
I generally don't ask my hitters to change their approach. IMO a hitter's approach is innate and personal-- dyed in the wool--and very closely linked to their confidence-level.
Often, our worst hitter bats 8th. Verbally asking my worst hitter to change his approach risks getting into his head and making him worser.
As mentioned, the take sign allows us to guide their at-bats--without getting into their heads the way it can if we verbally ask them to change their approach to fit the situation.
The above doesn't necessarily apply to a star-studded lineup where everybody is a very competent hitter (though of course some are better than others.) But IMO it applies to the typical HS team, where there can be a large difference in competence between the better hitters and the worser ones.