This is pretty typical. The big companies tend to invest in the little ones so that if the little one finds something good they can buy it and put the rigor of large scale manufacturing (and marketing of course) around it. If the little ones fail to find anything they can cut their losses without having the moral hit of laying people off.
For now: Pfizer has already got what they want: a license to produce a useful vaccine. Conversely BioNTech also got what they need: someone able to run a large trail and scale up manufacturing while giving them some profit.
In a few years both companies will re-evaluate their relationship. Partnerships can last for years at times. Other times one company is bought. Other times they go their own ways. All are normal and mean nothing, though if you are an investor each has different implications.
BNTX's market cap is pretty large already, it's unclear how desirable a buyout would be by a Pfizer-type company. It seems better to split profits / contracts.
