I understand why the wasteboard has to be surfaced with respect to the cutter.
Why does the shapeoko bed itself have to be level with respect to gravity?
Isn’t close, good enough?
I have seen the pics here of how people sometimes obsess over the table being level on a spirit level.
Not necessary. You could probably stand the machine on end or upside down and it would still work. Would just be more prone to missed steps.
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AM I missing something… gravity??
Installation irregularity, Yes
Varied Board Thickness: Yes
Workbench top irregularity: Yes
But Gravity? HUH? Help me help you by educating me about the weak force known as gravity…
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It helps to begin with a level surface — precision has to start somewhere, and this makes it easier to check if things are parallel/square — if the machine doesn’t start out on a level surface, then you have to choose something else as a reference plane and work from that.
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Gravity…
I think the reason is simple: Without another reference (you may have noticed that gravity is free and fairly well distributed - at least within the 3 feet the machine fits in) its much harder to actually make the wasteboard -and frame- flat, reducing the number of planes that need to be adjusted with more specialized tools. If leveled using a machinist level, you now have a flat plane wrt the rails. If you keep the base level (which is fairly easy once it’s leveled) you know the rest is a parallelogram (or at least very close to it) - and that’s a little easier to work with. If the base has twist in it, and it surely will, using it as any kind of reference and surfacing it flat is a waste of wasteboard , it’ll move as you adjust the rest of the machine, and will be very much twisted in a different direction by the time you get the rails right…and you’ll need to surface it again…but given that you’ve used that “bad” plane as a reference to get the rest right…the whole machine will be out a bit.
Anyway, gist is, a good level (check it first!!!) can be used as a perpendicular or parallel reference anywhere, not against another surface, which is awfully handy when you don’t have a couple parallel or perpendicular known surfaces. It’s certainly not the only way, but it does have some convenience. If you leave the machine leveled (I do not, level the workbench with blocks and shims, sort out the machine, and then remove the shims from under the bench), you can use a level to help with setups where you can’t get to the surface you want with a square, and use a level instead.
Do note that your garage floor is not flat, and generally has quite a bit of slope to the door (mine is about 3 inches over 4 feet and it sure seemed unsloped…it’s sloped!), and is generally a bit uneven perpendicular to the door. A shop floor (built as a shop) will usually be kind of wavy in both directions. Using that directly as a reference for “flat” turns out to be not so great at this scale.
A big surface plate would make it all pretty easy…but I don’t have one of those 
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In theory you want your base flat and lev. In reality the base tends to say in the middle on xxl. Reality is s mdf is subject to swelling and shifting. Reality your bed and spoilboard are seldom flat and square. Finally the reality is it does not make too much difference for most work. Level your spoilboard and don’t sweat the small inconsistencies unless a particular project demands more.
I’ve never checked if my machine/cabinet are level. Until today. Always assumed it was not level as it’s on a garage floor that building code says should be angled away from the house.
Anyway, per @mikep, check that level.
I’m in the level is not so important camp. Rigidity and parallel are what you want.
It helps when making everything square to give you something to reference, but after that I find it not so useful either.
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