There are a lot of contracts where the start and end date is non negotiable and already set in stone by some higher ups. You have to create a lot of documentation and feature list, give test cases for your work etc. Waterfall definitely works better in these type of work. Common at banks, pharmacies, governments... You can't really say at the end that some tasks did not make it to the last sprint but "hey it's in the backlog so no probs".