Do member functions that update fields have an implicit merge/upsert?

Assume I have a Type and a member function that updates one of its fields as follows:

entity type Lightbulb schema name "LGTBLB" {
	numTimesSwitchedOn: !int
	switchOn: member function() js server

function switchOn() {

Is the line this.merge() needed or redundant?

merge() is needed in this case.