From the Introduction:

Hello fellow adventurer!

If you like old school gaming and wanted psionic powers in your game, you found yourself in a rough position. Or perhaps you’re familiar with looking up a monster to discover a listing for psionic ability, only to discover those rules don’t exist. Perhaps you’ve decided to use the rules as written only to find a certain select set of issues with the rules. And at that, I’m being kind.

And yet the desire persists! Psionics! Strange mental powers! Psionics fits exceptional well with the old-school pulp gestalt of Dungeons & Dragons. The thought of great mental minds in conflict, an energy separate from magic, powerful, yet fraught with danger! So therein lies the reason for the creation of this supplement.