grammar question

Forget about the article "the" for a moment b'coz explaining it would kill too many brain cells. Ok, we left with "all of " vs "all", looks easy enough and now, the difference between them is only word "of ". The preposition " of " as we often use, always express a relationship a part of something whole. Adding "all" to "of " would fall into parts of many of the whole so to speak. Let's see example.

All my apples are rotten. 100% rotten apples (without indication)
All of my apples are rotten. 100% rotten apples but... the word "of " indicates an idea that there might be other fruits.

AA = all A's are rotten

(AA + BB + CC) => all of A's are rotten.

Answering to your question in specific.
(All the apples) would indicate nothing more
(All of the apples) indicates a possible part of a whole fruit basket.

Go bananas.. simplest word weigh tons in explanation.