This is something I've always noticed confusion about, so I'm glad you brought it up. Here is how I think about it.
Outcomes are a function of 1) process and 2) luck. For example, suppose you want to make at least $1,000/month from a side project within three months. That is an outcome. The outcome depends both on things you can control, and things that you cannot.
Intuitively, my first thought is "Why focus on things you can't control? Why not just focus on what you can?" Since outcomes depend partly on things you can't control, it doesn't make sense to me to focus on them. Instead, it makes sense to focus on what you can control: process.
However, I do recognize that focusing on outcomes might be a motivational hack. For example, if you know you have that goal of making $1,000/month from a side project, you might push yourself harder to reach it than you would if your focus was on something process-oriented like "put in a good days work every day". I'm sure there are other considerations I'm not thinking of, but to me the question seems like it's mostly about how much you want to use outcomes as a motivational hack.
The problem with outcomes is that they depend on luck.
The problem with process is that it is unfalsifiable. You can keep doing the same thing based on mistaken assumptions, and telling yourself that you simply had bad luck.
Perhaps the right approach is to focus on the process most of the time, but once in a while check whether the outcomes match the prediction, and update accordingly.
One other benefit of focusing on outcomes is that it keeps you focused. If you're just trying to "put in a good days work every day" then you'll probably waste a lot of days on rabbit holes that aren't actually helping you get towards your goals. They'll feel productive, but in hindsight they probably didn't get you closer.
I've been thinking recently about how to balance between process (how I get work done) and outcomes (what I achieve). I thought I'd ask the LessWrong community to see if anyone else has thoughts about this they'd like to share. I feel like both are important, but outcomes is a more long-term focus thing and process more of a daily thing. Outcomes are like long-running experiments for how you judge between different styles of process? In cases where it's hard to get reliable outcome answers, when failing at hard things or succeeding at easy things, or timeframes are long, or uncertainty high, it can be tempting to over-update on limited evidence. Is it then better to test process types on easier examples and then extrapolate to harder ones?