Why proof a baguette seam side up
Leaving the loaf seam side up makes it easier for armatures like me, to roll the dough onto a baking stone or floured peel and reduce the chance of deflating your loaf.   Also, sometimes the dough can bulge/blow out during baking.  Baking the loaf with the seam side down helps support the structure of the bread while slashing the top gives the loaf room to expand while cooking and provide help to control the possibility of bulging/blow out. 

The first pic shows expansion where it was slashed, while the second pic shows a blow out due to not enough slashes, deep enough or other reasons.  Imagine what it would look like if the seam side was up?  The loaf would look like it threw up.