Why so many letters?

(This is lesson 4 of the alphabet)

You may have noticed that there are 3 letters which represent the 's' sound, 3 letters for 't', and 5 letters for 'z'

And you gotta ask, why so many letters?  The short answer is that Pashto is f**ked up.  Why it still exists, no one knows.  Regional dialects can vary from valley to valley (and there are a lot of them, as Afghanistan is very mountainous), and even from village to village.  It got so bad at one point that sometimes a person from one valley couldn't understand a person from the next.  However, the advent of TV has universalized things somewhat, though different parts of the country pronounce things differently and people still have trouble understanding each other.  No wonder they've been fighting each other for thousands of years (when they weren't fighting off invaders).

But the real reason is that, since the Pashto alphabet is derived from Arabic, and Pashto uses a lot of Arabic words, especially when it comes to religious topics or higher education, they decided to keep them.  This is what Wikipedia has to say on the topic:

"Ten of the letters, ق ف ع ظ ط ض ص ح ﺫ ث, appear only in loanwords which are mostly of Arabic origin. Eight of them, ع ظ ط ض ص ح ﺫ ث, represent no additional phonemes of Pashto, and their pronunciation merges with other phonemes"

Basically, the pronunciation of those letters in Pashto isn't the correct Arabic pronunciation - for example, the letter ث in Arabic is actually pronounced 'th' as in 'tooth', but Pashto pronounces it 's'.