It's mostly just the fact that they're different and semi-rare. Sometimes they contain good items. I find Nuka Cola Quantums in them fairly often. The luncboxes themselves can be used as decorations. Other than they that can be made into bottlecap mines, which can be pretty nice.
I found a ice cold nuka quantum a couple of times in them. They are worth opening.
You can also find the robot model kits in them, fairy rare though
Hi quality adhesives drop is what I note them for.
http://fallout.wikia.com/wiki/Vault-Tec_lunchbox
I love that when you open them they make a surprise birthday noise. It's such a silly thing.. I wonder who thought of that? I also love when things that could never actually fit in that lunchbox pop out of it.
I may be imagining things but I swear a mini nuke popped out of one at some point.
Don't know about mini nukes but I know you can get fusion cores from them. What would be funny would be to make a mod that added really silly things to the list of spawned items. Fatman and giant teddy bears would be quite hilarious. (I was disappointed when I picked up a giant teddy bear somewhere and when I dropped him back at my home he turned back to a normal sized teddy bear again.)
You can manually place them as decorations using the "grab" feature and hope they'll stay put, but you'll risk possibly losing them forever.
I use that sound as my text message alert on my phone. I get weird looks, and the occasional person who knows... usually demonstrated by the quick "up nod" of approval.
I lost my Eyebot model that way, since one of the updates everything stays put though - even my toy alien which used to end up on the floor outside.... but I still havent found that damn Eyebot!