Because the spacing (or what you intend to do with the spacing when you get around to it) will potentially have an impact on what you can do with the posing and timing. This creates a chicken and egg scenario, the timing and posing needs to be done before the spacing, but the spacing is needed to inform the decisions you will make about posing and timing.
Experienced animators who have seen their spacing in action many times over form a mental image of what their spacing will be for a scene (not always how it turns out in the end, but an educated guess to use as a starting point) and then often without even realising it they will factor that into the poses they create. But for a student who has not yet seen much of the spacing they have produced in action this can pose a problem, I find that the uncertainty of how spacing will effect the animation can lead to a reluctance to embrace and practice it.
So an example.
But you can see I have decided to make him accelerate up into the final pose, there are going to be lots of inbetweens around the start of the scene and not many at all towards the end. In fact at the end of the movement he moves really fast, not because of the timing, but because the inbetweens are spaced out so that he covers the whole second half of the movement between two frames. As it is here we have heaps of time to absorb the movement around the first pose because there are lots of inbetweens close together there, but the end of the movement seems to pop because there are so few inbetweens that we can hardly take in the movement at all.
I know there will be lots of time to take in the anticipation pose because it is where he is moving lowly, so the pose probably needs to be towards the believable end of the range I'm shooting for in this movement. Anything strange will stick out because the viewer will have time to study this moment. The overshoot pose on the other hand will be quite different, the spacing will make the movement will be very fast at this point, this is where I can be a little more adventurous, viewers may not be able to study the pose, but I still want it to have impact.
On this still of the overshoot frame you can see that I have stretched out his body and arms a lot (obviously 'a lot' of stretch depends on the style of animation you are doing). Looked at as a still it is a very unusual pose, but when seen in motion you wont be able to see this, because I know what the spacing will be doing at this point in the scene I know this key is more about creating something that you feel than something you see. Knowing what I will do with the spacing informs what I do with the key. it provides a great place to play and be creative, you still have to test your work a lot, I'm not talking about anything so extreme that it breaks the continuity of the animation, but in some cases you would be amazed what you can do.
With some spacing added you can't really see the stretch at all, but hopefully you can feel the impact of the overshoot. If I had more time, I'd refine it further (and go on to add overlap etc etc), what I would be aiming for is the most impact I can get out of the poses without it seeming to pop (loose its continuity).
I remember seeing a key frame of Rafiki in Lion King 2 where he is spinning around with his arms out as he sings and for one frame his hand almost completely fills the shot. Not only is the hand completely blocking the view of the characters face, but the hand is all stretched out. I was a newb inbetweener at the time and remember thinking that it must be a mistake, but when I saw the scene playing later I realised you couldn't even see the frame because the spacing dictated that it was moving very quickly at that point, but you did get a distinct feeling of the hand swooshing past the camera. Its about 2:03 into this video, see if you can spot it :P
Spacing is a key factor in how you block and time out your animation. Its hard to taking into account when you are first learning because you can't necessarily see how it will help. The solution is to produce, but keep an open mind, be adventurous about how you uses spacing. Explore the extremes, find that point where a scene breaks and then come back a bit from there. How much can you push the inbetweens around between two keys and what impact will that have on the keys themselves. I think a good animator knows where the edge is, and isn't afraid to step over it every now and then to see what happens.