From discussion before the Wuhan lab was built: "worries surround the Chinese lab, too. The SARS virus has escaped from high-level containment facilities in Beijing multiple times, notes Richard Ebright, a molecular biologist at Rutgers University in Piscataway, New Jersey. Tim Trevan, founder of CHROME Biosafety and Biosecurity Consulting in Damascus, Maryland, says that an open culture is important to keeping BSL-4 labs safe, and he questions how easy this will be in China, where society emphasizes hierarchy. “Diversity of viewpoint, flat structures where everyone feels free to speak up and openness of information are important,” he says." - https://www.nature.com/articles/nature.2017.21487
The link above is a fairly readable summary of a conference in which academics debated the need for gain of function research. As you can see, this was not a settled topic, and even some of the supporters of GoF research admit doubts about its utility.
To summarize the paper I posted:
1. Gain of function research, broadly construed, also includes adapting viruses to live "in culture", i.e. in a dish of cells. That's a fundamentally useful and necessary part of research, so a blanket ban could unnecessarily complicate basic research.
2. GoF is useful for evaluating the potential impacts of mutations observed in the wild. Like, we see a particular mutation circulating, but its significance is unknown, so we introduce it into a model virus in a lab to test what impact it has. This helps us decide which strains we should choose to develop vaccinations for. This can also be done prospectively, GoF can identify particularly nasty variants, which we can then prioritize for more intense intervention if they're observed in the wild.
3. There's a basic research interest in understanding the molecular details of how viruses infect humans. GoF allows researchers to probe this much more easily than the random sampling we get from wild viruses. This research has led (in part) to basic insights in virology, like the role of the furin polybasic cleavage site in coronaviruseses in increasing virulence.
Personally, I think that's a bunch of bs. The stuff they're doing to these diseases isn't likely to occur naturally. I suppose you could make the argument they're preparing to stop some possible bio-weapon that might be created.
I also think a lot of it has to do with money. In Pfizer's case they're wanting to get the jump on vaccine sales down the road. If they're putting that much money into creating a vaccine for a variant created through gof... well that opens a lot of doors for nefarious activity and the possibility of someone intentionally releasing it goes up.