Dave beat me to it, but since I've type this here it is anyway
SketchUp's Layers do not work like CAD's.
Layers only change their objects' visibility.
They do not 'freeze' the objects.
So they do not stop some changes to objects on visible layers affecting connected objects on hidden layers - typically 'geometry'...
If you draw a rectangle and put its face on one layer and its edges on another layer and switched off the face's layer... then you deleted a visible edge and switched back on the face's layer, you'll find that the face has gone too - because it can't exist without all of its edges.
That is why you should ALWAYS create your raw geometry [lines/faces] on Layer0.
Always have Layer0 as your Active-Layer.
Then you make groups or components of that geometry and assign layers to those groups/component-instances.
That way the Layer's visibility hides the 'container', and since there is no connected geometry spanning different layers you will not fall into that trap of assuming CAD-like layer-behavior...
Drawing elements like Text and Dims can be given their own layers and switched on/off, but be aware that changes to visible entities could affect them in subtle ways too - e.g. if you erase the object to which a Dim is attached its color might have changed when you next see it [a Model Info > Dims setting for 'orphaned' dims], and moving an object might unexpectedly stretch the leader of its attached Text when its layer is next made visible... However, it IS common practice to assign separate Layers to Text and Dims to allow you to toggle their visibility on/off - there are even some plugins to automate that for you...
