Why does one need to call a @Modifying @Query(..) repository method within a @Transactional method?

If I try to call @Modifying @Query(..) repository (not annotated with @Transactional) method with simple UPDATE inside service method without @Transactional too - I get

org.springframework.dao.InvalidDataAccessApiUsageException: Executing an update/delete query; nested exception is javax.persistence.TransactionRequiredException: Executing an update/delete query

I dont undestand why I need to use @Transactional in this case: there is only one simple update (which is transactional at DBMS level). Why?