Why Batman Always Needs A Robin
1. It Stops People Following In Batman's Footsteps
What makes Batman such a popular pop culture figure is that the Dark Knight is far from your regular insta-smiling, baby-kissing, granny-hugging hero who always does the right thing. Far from it, for Batman is famously a more brooding proposition who will often do whatever it takes to get the job done - so long as it sticks within his 'no killing' policy.
One of the great benefits of Batman having a Robin - or any young sidekick, for that matter - is that Bruce Wayne can help to steer that person away from following in his own vengeful footsteps. Bruce is clearly a troubled, tormented soul, and his presence as a mentor to similarly troubled youngsters can help to stop creating Batman-lite heroes brimming with pig-headed angst.
Sure, Dick Grayson has stepped up to become Batman at several points, but Dick's Batman was completely different to Bruce's - and Grayson's ascension to the cape 'n' cowl was largely done to keep the figure of Batman out in the public eye while Wayne was out of the equation.
By taking impressionable, troubled teens under his wing and making them Robin, Bruce Wayne is able to try and help these young heroes avoid making the mistakes that he has made during his tenure as the Caped Crusader.
Even in the example of Tim Drake - the Robin viewed as the true natural successor to the cowl - we've seen how Tim himself doesn't want to become Batman, and we've likewise seen how the future in which Tim does become the Bat doesn't turn out all that well.