In a sufficiently large universe, everything that can happen happens somewhere, but it's clearly not an even distribution. Flip a quantum coin a hundred times, and there have to be some versions of you who see a hundred heads, but they're so vastly outnumbered by versions of you who see a properly random-looking sequence of heads and tails that it's not worth thinking about: it mostly doesn't happen.
When you write a computer program, or build a bridge, or just think something, we might prefer to take the viewpoint that you're not creating anything so much as you are instantiating that pattern locally, thereby increasing its measure in the multiverse: there might be other ways for that program, that bridge, that thought to come about somewhere, but it's getting some of its support from you.
Decisionmaking is about exerting some control over the distribution of measure over patterns in the multiverse: agent-like patterns select actions so as to allocate measure to their preferred patterns. Maybe there are some versions of me with an ice-cream cone that form by chance, or are deliberately created by alien civilizations investigating how humans respond to ice-cream, but if I get an ice-cream cone, it's mostly because humans evolved and then developed cultures which domesticated dairy cows and cultivated sugar and so on and so forth until eventually I was born and grew up and put effort into acquiring ice-cream. When you decide, you help determine the distribution of what happens to the sections of the multiverse that depend on copies of your decision—choose carefully.