I have this theory, it goes something like this:
The more you know about what you want from life, the happier and more centred you will be.
This is why it is important to sit down and think about what you want, and one of the best ways in starting that process is looking at what you don't want in your life. Everyone has some idea about the things that make them unhappy, the things that haven't worked in other relationships and typically have a mental list of things that they certainly don't want in the next relationship (whatever sort it may be).
And people have the opposite too, the things that did work, the things that do make them happy, the things that they do want to find... and see you're almost there already (at least for relationships) on a path to finding what you want.
So, why is it important? I've found, as an available woman dating on OkCupid, that people are attracted to others who know what they want, who have that added degree of certainty about themselves, and who appear confident because they've decided things about themselves, their desires and to some extent how they fit in the world.
Which is not to say that I know everything about what I want, but I know a lot. I know a lot about what I want from relationships, I have a fair idea what I want from potential employers when I finish my degree and start looking for work, I have a fair idea about how I want to interact with people and how I would prefer people to see me.
Why is this important, and what does it have to do with polyamory? Figuring out exactly what I wanted has taken me quite some time to figure out, and evolves as I personally grow and change. What I think I may want today will gradually crystallise over time and may be quite different in form in 5 years time.
But back to polyamory. If you don't know what you want, then it is very hard to ask that from one partner, let alone many. In my shoes certainly, polyamory has taught me that to get what I want and need I not only need to be confident enough to ask and/or take it, but that I need to know what it is. Monogamy gave me greater freedom to be slack. I could poke and prod James from time to time and he'd figure out what I wanted and I'd be happy with that.
Add additional partners and the resorting of how your world works (in your head) and suddenly
there were a lot of different parameters for me to consider. James knew me thoroughly - he understood me and could answer my needs before I knew they existed (and yes, I do know how blessed I am to have such a wonderful relationship). A new partner required me to start verbalising some of my wants and needs.
My former partner expressed his frustration with me in relation to people who didn't know what they wanted, and I didn't understand him as well as I do now (and not even he really knew what he wanted). Doing someone else's thinking for them, second guessing them and constantly trying to read between the lines is hard and tiring work.
Of course most of this goes back to communication between you and your partner, but to communicate what you want and need, you need to know what those things are. Of course, this holds true for monogamous relationships as well as polyamorous ones.
In summary, knowing what you want makes you more confident and means that you can start attempting to source those things. You can start talking to people who are close to you and ask for their help in attaining those goals, personal development and experiences. For me, knowing what I want has meant that not only am I more confident, but I'm also much happier as I'm getting what I want and need.