Why would two mating parts with a bolt ever need a clearance at worst case scenario?

[Baratheons]

If I have two surfaces with holes, and I calculate at worst case scenarios that they have clearance of 0 after an installation of a bolt, is there an issue?
Let's say I have a .25 bolt, and my holes at virtual conditions are .255 and .265, I then have a worst case clearance of .010. But why not make the virtual conditions of both or one hole so it is closer to the diameter of the bolt? Why leave residual clearance, of in my example .010? Why not make it so VC = diameter of bolt?

Does my question make sense?

 

[3DDave]

The reason is that the datum features are not the same as the theoretical datums and therefore the mutual alignment between the parts will not be identical to the alignment found in inspection. It can be easier to leave a small amount of clearance than to make all the datum features perfect. As a practical matter, there are some cases where negative clearance is OK because one can pound the fastener into the hole with a hammer. It's up to the designer/analyst to decide the worth of the risk.

 

[chez311]

Just to put what 3DDave said in even simpler terms: Real world conditions will never match the ideal conditions found in your inspection room.

Even if your parts pass inspection at the outer limits of their tolerance range, if you end up with a zero (or near zero) clearance condition at assembly it could still cause binding which may be difficult to assemble without added force or require the operator to adjust alignment continuously until an orientation is found which allows the parts to mate together. The problem is compounded when the parts are especially heavy, large, or both and/or require the operator to hold them at a difficult angle/orientation. All of these things cost money and time and cause operator strain on a full scale assembly line (or even an inability to assemble the parts even if theoretically they should fit) - most of which could be avoided by building in a little clearance.