I didn't know they had manufacturing capabilities. Looking at their manufacturing page, they do have GMP manufacturing facilities, but it looks like they were built around their oncology pipeline, which would be customized for each patient and low volume (as you mentioned).

https://biontech.de/how-we-translate/manufacturing

And unless I'm mistaken, they don't have any approved products on market? No doubt they have experts in house, but when you're racing to get a product to market, it can be very helpful having someone like Pfizer helping out who has successfully gotten 70+ products approved by the FDA (including several vaccines).

And interestingly, you'll even see these partnerships across big pharma. I worked for one company who was interested in bringing a gene therapy in house until we did our due diligence and realized holy shit, we know nothing about gene therapies. And this was a top 5 pharma company.