It's a matter of practice only. There is really no logical explanation for those things.
This preposition cannot be translated to English, since "need" doesn't use it.
If you look into English, you will see some verbs need that too:
- I listen to my father
- I hear my father
Why does one use the preposition and the other don't? Well....there is probably no answer for that.
One way to know when starting is to look in the dictionary.
There, verbs are classified in:
- "intransitivo" (no objects) - choveu hoje (it rained today)
- "transitivo direto" (objects without preposition) - comprei um carro (I bought a car)
- "transitivo indireto" (objects with preposition) - gosto de música (I like music)
- "transitivo direto e indireto" (both objects) - enviei 'uma carta' 'para minha mãe' (I sent a letter to my mother)
Oh, now I get it. (Sorry for not giving a correct answer the first time...)
The sound of the first letter in 'eles' and 'elas' (or in 'ele' and 'ela', for that matter) is not the same. In 'eles', it has a "closed" sound, as the 'e' in 'they'. In 'elas', it has an "open" sound, as the 'e' in 'bell'.