Why is "el" sometimes used as an indirect object pronoun?

Why is "el" sometimes used as an indirect object pronoun?
 

I just had my pimsleur tape on and I heard "he hablado con el". But In this sentence, el (the person I have spoken to) is the indirecr object (the person who the speaking was for). So why isn't it "Le he hablado" or "con Le he hablado"?

It's part of a prepositional phrase. with a function similar to an indirect object pronoun.

When 'hablar' is followed by 'con', the verb is being used intransitively (no indirect object). A prepositional phrase provides the 'about what' details. Another prepositional phrase provides the 'with whom' details.

Hablar con alguien de/sobre/acerca de algo (con alguien: complemento preposicional)


The transitive form allows the use of an indirect object pronoun, but the 'about what' details are still provided by a prepositional phrase.
Hablar a alguien de/sobre/acerca de algo (a alguien: complemento indirecto)

Why is it different for decir, then? Why is it lo digo...why is decir not intransitive?

That a particular verb is transitive while another isn't really has no bearing here.

Hablar and decir are not synonyms, so can't be used interchangeably. There's no reason to compare the two syntactically, either.

Hablar is used intransitively anytime it means 'communicate with someone through words', but it can also be used transitively when you refer back to something to be communicated and use a pronoun to do so. Por ejemplo: No quiero hablar de eso ahora. Cuando venga mi marido, lo habla con él. ('lo' refers back to 'de eso')

Decir is used transitively whenever it means 'communicate something with words', but it can also be used intransitively when it means 'inform or give notice'. Por ejemplo: Dime de mis hijos. (note that no direct object is used)

Many verbs classified as transitive can also be used intransitively.

Then on a related noted, why is it escribirle for "write to him "? Shouldn't it be escribir a el? Why isn't escribir intransitive

You may say 'escribir a él'. Escribir is transitive (what is being written is the object). Escribir may also be used intransitively. Por ejemplo, "Ella escribe bien." (note there's no object)

Again, many verbs classified as transitive can also be used intransitively. This is same as in English.

Sorry to go on about this, but if what is being written is the object, similarly, wouldn't what is being talked about in "hablar con el" be the conversation. Just like "write to him" is "escribe Le" because it's 'write (the letter) to him', why not "habla Le" because it's 'talk (the conversation) to him'?

"Escribe Le" and "habla Le" are incorrectly spelled.

Escribir takes an indirect object - escribirle a alguien algo. So does hablar - Hablarle a alguien de/sobre/acerca de algo.

Your original question was not about the latter form. It was about 'hablar con'. That is different, as documented above.