How do you get from "putus" (which I assume is the root word?) to "memutuskan"?
From my understanding, "putus" means to disconnect something, cut ties, or cut off / break up something causing some sort of separation? Like a relationship, or a cable, or a thread, etc...
So if "memutus" is to break (or whatever), how does it get to "memutuskan"?
“To discontinue the thinking process at a certain point even though it could go on if you wanted it to” perhaps? Maybe makes it easier to see why it needs to be followed by “untuk.” It's like “We break off the thinking process in order to do so and so,” you know. According to Google Translate, “memutuskan” also carries over the original meanings of the root: “break off, disconnect, sever, cut off [, etc.]”
Come to think of it, there are Germanic words for “to decide” that have a comparable etymology: beschließen in German, besluta in Swedish, etc. Schließen and sluta mean to close/shut or finish/conclude. Be- as a prefix has a causitive meaning, so it's like the verb literally says “To make it stop.” Doesn't me- -kan also have a causitive meaning? It's basically the same verb reinvented independently half-way across the world! Hell, even English “decide” comes from Latin de- ('off') + caedō ('I cut') = “I cut off” (cf. German entscheiden).