Why do we have double letters?

Solution 1:

I'm not a native speaker but I see it this way:

Two consonants in a word give us a different pronunciation like in:

• apple and aple are different in pronunciation [ˈap(ə)l] and [ˈeɪp(ə)l]
• little, better (double t sounds like d)
• ladder, bidder (without the double d, it wouldn't be [a] but [eɪ] instead)
• well (without the double consonants I think it wouldn't be [w], but [v] instead)
• sunny (without double n would be pronounced as [sjuni] instead of [sʌni])
• happy (without the double p would be [hāpi] instead of [hapi])
• watt (without double t would be [wat] instead of [wɒt])

As with vowels i think it will be:

• feet and fit are differently pronounced [fiːt] longer i and [fɪt] shorter i. Same works for teen, beetle, tree.
• hoop, spoon have a longer sounding u. [huːp], [spuːn]. You cant' write spun [spʌn], hup [hʌp], because they are pronounced differently and there is no long u in English as a letter.

Solution 2:

The double consonants in well and apple are making the vowel short as in pest and fast. If the p were not doubled in apple, the word would rhyme with maple; compare apple with dapple.