Hello Arabic speakers and learners!

I have a quick question about the Arabic language and would be grateful for anyone's insights:

If the long Vowel ‘alif | says aa why would it even be necessary to add two extra short vowel strokes to the word? Do they just exaggerate the pronunication? Maybe one for between the n and the h as: n ' h but why add an extra when | is already sounding aa ? Jamiil clarifies things and juluus even more so. I guess then, that each vowel of a double vowel as uu aa ii need to be sounded.

Thanks in advance!