— 1.1.9: two sounds are सवर्ण when their place in the mouth (आस्य) and their effort (प्रयत्न) are the same.
Now read the sandhi rule that depends on it — 6.1.101 , "when an अक् sound meets a सवर्ण one, a single long vowel replaces both". That rule cannot be applied at all until you can say whether two vowels are savarṇa. And you answer that by asking where in your mouth each is made.
That is the whole of this page's practical payoff: the same two vowels behave differently depending on whether they are made in the same place.