How do pretrained models using SQUAD dataset work on an any other dataset?

I see in some Kaggle contests people have used models pretrained in SQUAD dataset for building QA systems for the dataset given in the contest. How does this work? How can a pretrained model in a completely different dataset be used for any other dataset in building QA systems?

Pre trained models make sense to me when used for image classification because same objects in different images may have same features. Similarly it also makes sense to be used for sentiment analysis because there are only finite number of words for each sentiment.

But how can a pretrained model be used for building QA systems?