Okay yo check it morons
There are several reasons why a bro would create a test tube virus
One is to understand bacterial evolution and DNA, which can help us predict how bacteria in the body tend to change
With this, having bacteria suddenly do a heelflip and render our medicine ineffective will be less troublesome

Another thing is because vaccinations are easily produced from one based off a similar disease
If a disease similar to this were to appear, we would have a much better chance at stopping it before everyone is f***
This also tends to help with diseases that are already problems

The best one imo is so I can use these diseases to cleanse the gene pool when I take to the throne