After first injecting himself at Oscorp, Curt created his own lab in the sewer system later in the movie. If he was the head of the lab at Oscorp, couldn't he have just kept everyone out of the lab (he had already done this previously) and done all of his testing there, instead of doing it in the sewer?
The Lizard often retreats to the sewers / lives in the sewers in his appearances in the comic "The Amazing Spider-Man".
One of the more famous Lizard storylines The Gauntlet has him trying to get humans to act like Lizards / turn them into lizards. He does this whilst hiding in the sewer.
Practically this is useful for a reptile who likes wet / humid conditions whilst also eliminating to some degree Spider-Man's main asset which is his agility given the cramped confines. It also allows him to navigate the city quickly without drawing attention to his form.
The current story arc in the Amazing Spider-Man which starte in issue 688 (I think) features the Lizard who is found living in the sewer.
So I think the film was trying to replicate the character from the comic for those comic fans out there, like myself.