It is not yet clear the extent to which coronavirus worsens influenza; there is simply not enough data at this stage on people with co-infections. But some things always hold true. Having two infections can seriously hamper your immune system. If you already have the flu, your immune system will not cope as well against coronavirus versus someone who doesn’t have the flu.
Also, people with the flu present to their GP or local hospital more frequently than those without the flu, putting them at risk of other infections, such as coronavirus. And, of course, if there are fewer people sick with influenza, the medical system at large will be better place to spend vital resources on combating coronavirus.