What is the function of the Aleph in كَتَبُوا?

Have you ever thought why we spell كَتَبُوا (they wrote) with an Aleph at the end?

When a conjugated verb has a و at the end, and when this و is preceded by ضَمّةٌ or by فَتْحةٌ, then the ا is often used (and written), particularly in the plural of verbs.

Let's see some examples:

This Aleph, in itself superfluous (Aleph otiosum), is in­tended to guard the و. The Aleph protects it against the possibility that the و could be separated from the body of the word to which it belongs – and thus could be mistaken for the word and (i.e., the conjunction وَ).

For this reason, this Aleph is called أَلِفُ الْوِقايةِ, the guarding Aleph, or الْأَلِفُ الْفاصِلةُ, the separating Aleph.

Note that there are other explanations as well, but most of them share the idea of guarding and protecting the و from being mistaken for other letters or from being absorbed.

Excursus: In which position will you never find an Aleph?

The Aleph can never stand at the beginning of an utterance.

The Aleph (ا) is a sound of prolongation after فَتْحةٌ (=“a”) and can't carry any vowel – which makes it different to و and ي since they are capa­ble of that. The Aleph, although mostly unwritten, al­ways has a سُكُونٌ on it (اْ). In Arabic, you can only start an utter­ance if the first let­ter has a vowel on it. Thus, you can't start a sen­tence with an Aleph.