Seam Sealer in engine bay?

[kmoses]

I'm prepping my engine bay for epoxy primer and paint. I know Fiat did not use seam sealer where the firewall meets the sides and where the sides meet the rails. I have worked on old German cars and they seam seal every nook and cranny in the engine bay. Is there a reason not to seam seal everything with a gap? My thinking is that it would help keep water out and prevent rust or am I not grasping Fiat's logic for not sealing things.

[RRoller123]

keeps engine fumes out too.