An automatic 3-of in any HB deck that isn't built around a specific agenda structure. Accelerated Beta Test provides what everyone wants in a game of mixed skill and luck: variance control.
In a game where you're winning or keeping pace with the runner, ABT is a blank 3/2 agenda, which is great. You always want to be able to fast-advance or never-advance out 2 points. In other words, it adds consistency to the game.
In a game where you're losing, you can fire an ABT to increase the effect of randomness on the outcome. You might hand the runner points and lose anyway, or you might drop a Heimdall 2.0 and a Tollbooth and suddenly make your scoring remote unbreakable.
That's only the value of the card on its own. Other reviewers have touched on combos with Biotic Labor, The Foundry: Refining the Process, Team Sponsorship, and NEXT Design: Guarding the Net.
If you're building your first corp deck, start with Haas-Bioroid: Engineering the Future and 3x Accelerated Beta Test. You won't be disappointed.