Hi,
accourding to the engine, the invisible objects are used by engine and he reserve them some memory.
try to put a bullet sprite invisible and fire it, you will see that he do the job !
i'm not a Construct 2 developer, but i'm a programmer and i know that Construct 2 is a normal software that is built with a C++ or whatever language that uses variables or memory.
in coding languages, the compilators reserve memory for variables even not used by user.