There are two main reasons for using a word order other than normal in Finnish: putting the verb or the object first emphasizes it, and putting the subject or the verb last indicates it as something new to the reader or the listener.
For example, Sai poikani kalan emphasizes the predicate. This is typically used to answer back when some doubt has been presented. Thus, the sentence repeats something that was said earlier, but in an arguing manner. The first word may be additionally emphasized with the pA suffix: Saipa poikani kalan. Here the pA suffix also makes the sentence arguing against an expressed opinion, not just against assumed doubt.
Word-like suffixes like pA have varying usage and need to be interpreted in context. When appended to a predicate that precedes the subject, they can express “argumentative emphasis” as described above. More often, however, they have an special role in making the start of a sentence smoother, somewhat like a sentence start like “You know,” does in English. It would sound abrupt to say Sattui aamulla hassu juttu (A funny thing happened in the morning; literally “Happened in morning funny thing”), but a pA suffix makes it normal: Sattuipa aamulla hassu juttu. If the word sattuipa is pronounced emphatically, the sentence would be arguing; normal pronunciation makes the sentence a normal statement.
The emphasis expressed by word order is generally not just stress emphasis. For example, Kalan poikani sai does not simply make kalan more important; it also carries implications. This sentence implies that my son caught something and clarifies what he got—for example, a fish, not a boot. In English, we might say “It was a fish that my son caught”.
Putting the unknown part last typically relates to situations where other parts of the stament have already been presented. For example, when you show a fish to someone and perhaps make comments on it, you might add Kalan sai poikani. Here the fish is assumed to be known, and it was apparently caught by someone, so the new information is who caught it. In English we would need to say something like “This fish was caught by my son.”