Halliday & Matthiessen (2014: 309):
The entity or event which is said to exist is labelled, simply, Existent. In principle, there can ‘exist’ any kind of phenomenon that can be construed as a ‘thing’: person, object, institution, abstraction;
but also any action or event, as in is there going to be a storm?, there was another robbery in the street. And here the ‘existential’ merges into the ‘material’ type of clause: there is little difference in meaning between ‘existential’ there was a robbery and ‘material: creative’ a robbery took place (note the present tense a robbery is taking place).