Adjustable rockers necessary??

Probably because something has been milled. When you machine the heads or block, the pushrods are now effectively longer and depress the lifter plunger further down with more preload. Without some sort of adjustment, this cannot be corrected.

IMO, the factory 273 rockers, when you change out the stock adjusters for high quality ones, are some of the best pieces available for the small block, barring something from perhaps Crane, Hughes, or Harland Sharp. IMO, they are that good.