I'm wondering: what exactly is this virus research for anyways?

Whether or not this lab leaked the virus and caused the pandemic, I'm wondering why they are interested in engineering new variants of SARS-like viruses in the first place?

Are the detractors of the lab-leak theory saying "Yes, they engineered plenty of coronaviruses in the lab, there's just no evidence that any of them leaked out" or are they saying "No, this lab was never trying to engineer new coronaviruses"?