If I understand correctly, in puhekieli one uses sinä and minä more than in kirjakieli. Apparantly, in puhekieli people like the redundancy of sinä even though the olet already points to the second person singular. P.S. The same goes for the first and second person plural.
In puhekieli they are needed because the verb is conjugated differently and therefore loses accuracy if the pronoun is left out. For example me olemme = me ollaan in puhekieli, the verb is in it's passive form and could get confusing if "me" is left out. Also the singulars get so short in puhekieli that keeping the pronoun intact is needed for clarity. Without the pronouns talking can easily get difficult to understand when the only difference between different persons is one letter: Olen=oon, olet=oot for example. And when asking questions oonko(am I), ootko(are you), simply too similar to use without the pronouns. In written text this problem does not exist, hearing a one letter difference in a lot harder than reading it.