The basic idea is to reduce the chance of achieving a low quality end result. This generally falls under composition.
Composition:The nature of something's ingredients or constituents; the way in which a whole or mixture is made up.
There's an old saying: Well begun is half done. This is quite apposite in the field of art as we've all been there at some point; looking at a painting we've been working on for a good few hours only to step back and realise... 'This is shit.'
Now there are a lot of reasons for this, lack of technical skill aside. Usually, if not most of the time it's due to a poor composition, that is, a poor choice of shape placement or the wrong colours etc. So many artists all around the world waste SO many hours on failed painting, paintings which they would believe they failed half way through when in fact, it's probable they failed it before they even put brush to canvas. What do I mean?
The difference between a compitent artist and an incompitent one is simply the ability to hold off on getting started on a piece to simply step back, look and study what you want to paint instead of rushing into it because you have this GREAT IDEA and want to get it down ASAP!
We've all done it - have an idea in our head, be it a creature concept or a super-kewl robot, and upon getting it down on paper we convince ourselves it's DA BESTEST EVAH! possibly taking it to a final piece there and then.
And with absolute conifdence I can say that it IS NOT the greatest it could be. The reason for this being that you can always push it further, add extras or take the crap away that doesn't make much sense. Even things like the characters expression or number of fingers can be changed out, the temperature of colour to set the mood and invoke certain emotion in the viewer and so on.
AND THEN he starts the final piece. Safe in the knowledge that he's pushed it as far as he can without wasting too much time on planning. As there comes a time when you have to say 'enough planning' and just get on with it due to deadlines approaching.
Really, nothing should be left to chance. I'm going to quote another artist here, again it's about composition though it's easily translated for game development as a whole:
Creating a good composition can be challenging but fixing a bad one can be frustrating – if not impossible. -Ian Roberts, Mastering Composition
Simply put, it's optimal to spend a lot of energy in the beginning getting it right as appose to jumping onto the production too soon only to hit a wall that's either really difficult, and time consuming (time is money) to get around. If it's even possible at all.