Data manuals are a necessity. Besides load data, they provide dimensions, test velocities, and some discussion of the caliber being loaded and it's characteristics.
But where I really enjoy having some decent load manuals is when I'm searching for data on a caliber I know nothing about. What's the difference between a .17 Ackley Bee and a .17-222 ??? The manual has the answer, not just in load data, but in specs for the cases, velocities, range of bullet weight and more.
But too, there's a lot of data available online (although I don't trust "folk" forumlas and worry about transcription errors). Most component/powder mfgs. have reliable data online.