The older, less capable systems had it because they were not fast enough to model realistic combat in real-time. Turn-based combat is extremely unreal. The essence of combat is speed and reaction. Fighting battles is not like playing chess. Also, please make a distinction between adventure games and RPG's.
Many people assume that the two are similar in design and purpose.
They are not. Adventure games stress exploration and puzzle solving. Playing an adventure game is like reading a book. RPG's,however, stress realism and completeness. In there, you find detailed instructions on anything from building castles and siege engines to probabilities of getting lost in unknown terrain.