Tails serves as Sonic's character anchor in keeping him from becoming an arrogant self-absorbed 'hero who always saves the day' by reminding him that, while yes, some people can keep up with him, almost no one can actually catch up as well as giving him a personal reason to continue fighting evil

And Sonic serves as a daily reminder to Tails that murder is both immoral and illegal and that he shouldn't resort to it in any given moment Sonic is not looking