Take these two scenarios:
Optional<Foo> opt = ...
Foo x = opt.orElse( new Foo() );
Foo y = opt.orElseGet( Foo::new );
If opt doesn’t contain a value, the two are indeed equivalent. But if opt does contain a value, how many Foo objects will be created?
P.s.: of course in this example the difference probably wouldn’t be measurable, but if you have to obtain your default value from a remote web service for example, or from a database, it suddenly becomes very important.