Welding...
In general, without welding separate faces, or separate unwelded parts, a mesh can be seen as completely separate pieces by an application. Smoothing may not carry over from one mesh to another, past the unwelded seam. So, instead of a smooth piece, you can get a sharp edge, or severl faces that just won't smooth out, even if the angle is within tolerance.... and you can apply smoothing factors till you're blue in the face, the mesh won't smooth...
(BTDT in max....)  

I'm sure there are other reasons for welding...This is just one example that threw me for a loop in my beginner days (not that long ago) ;)

Pakled... the answer to your question may depend on the application...  Often the bottomd of triangles or quads are shaded a bit darker, or not seen at all, as if there is a hole in the mesh.
Max (and I guess other more advanced apps) can actually show you an little icon on each face.The icon tells you which way is up/down... Then you have a tool to go and flip faces as needed.  HTH!

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
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