A valid reason is when there is other software running on the same server that needs some of the memory SQL server has, but that only runs a few times a month.
For example my wife has a SQL server based system used by 3 people including her to process a very large dataset, they do lots of ad-hock queries, but a few times a month they have to run a calculation engine that is on the same server, and accesses the database. The calculation engine needs memory. They don’t have a DBA, they can’t get funding for more hardware, even if they could, the IT department (who know less about SQL then the accountants do) would take months to setup new hardware, and a reset of SQL server lets them do their jobs as accountants. (The transactional system is separate.)