It’s like the difference between observe and reactive. One is intended to be run when some reactive variable is “triggered” and is meant to have side effects (observeEvent), and the other returns a reactive value and is meant to be used as a variable (eventReactive). Even in the documentation for those functions, the former is shown without being assigned to a variable (because it is intended to just produce a side effect), and the latter is shown to be assigned into a variable and used later on.