Bentley brings many skills to the table for the Cooper Gang and is an integral part of their success. Bentley creates many gadgets used by the Cooper Gang throughout the series, and creates the plans for major operations and heists that the gang must carry out. Bentley is the coordinator who helps each member come together to carry out a common goal.
However, perhaps Bentley's most impactful skill is his knack for hacking. Throughout the Sly Cooper Series, you can play as Bentley and "hack" with him, breaking down barriers for the gang that would otherwise be practically impossible to get through.
It is for these reasons that Bentley is considered the "Brains" of the operations.
A major turning point in The Cooper Gang's adventures is when Bentley suffers a major injury to his legs, preventing him from walking again. However, Bentley not only comes back from this, but creates modifications to his own wheelchair to make him one of the most fun characters to play as in the Sly franchise. Among other abilities, his additions to his wheelchair enables him to throw and place bombs, hover through the air with a built-in jetpack, and the ability to turn on "turbo" mode for quick getaways.
Bentley goes on to make a monumental invention and creates a time machine to enable The Cooper Gang to go back in time in Sly Cooper: Thieves In Time.