Seam welding virtues

This is probably a pretty newbie question, but what are the advantages of having all of the body and chassis seams welded instead of using seam sealer? I occasionally hear people talking about having their entire car seam welded, but never an elaboration as to why.

Also, if I get this done on my Ghia Coupe project how many hours would it normally take a pro welder to do the pan and body? My welding skills suck big sweaty camel balls, so I'd need to farm it out.

Thanks!