Even though it is insecure, it still exists! Can someone tell why exactly do we need Aggressive mode? What is it that Main mode cannot do and Aggressive mode can (other than speed)
Well, the original reason behind Aggressive Mode was, in fact, performance. Specifically, in used fewer round trips to establish the connection than Main Mode.
The original designers of IKE were well aware that Aggressive Mode had lesser security goals than Main Mode; however they did not know how important those goals were, compared to how important connection set up was. Hence, they included both options.
When they reworked IKE in IKEv2, they reconsidered the options; they decided that the trade offs inherent in Aggressive Mode were a bad trade off (and telling people to support multiple 'modes' was also not a great design); they were able to rework things to get the performance advantages of Aggressive Mode, and keep the security goals of Main Mode.