When you travel, especially internationally where you're probably carrying the laptop in a hand bag then the extra weight starts to be an annoyance. A lot of walking around and 0.5kg savings makes a difference.
Well aluminum also has the nice property that it's a good thermal conductor, it definitely has functional advantages over plastic. It also has a good specific heat capacity, so as well as conducting away heat via the chassis, it can allow longer bursts of high power usage before temperature limits are reached.
I carry two. And power leads. And clothes for upto 2 weeks (three at a push). And a 10 port mikrotik, a few cat 6 cables of various lengths, a supple of lc-xx sm fibres and an sfp. Sometimes a pi or two as well.
0.5kg makes a significant difference when you're rock climbing because you have to pull that weight up vertically with you. If 0.5kb makes a significant difference while walking around, I suggest that you join a gym instead of buying thinner MacBook.
Marginal gains. A lighter bag, a lighter laptop, less clutter in such bag, can add up. If you're carrying things in a backpack, you can carry heavy loads without an issue, except then in hot buildings you end up with a sweaty back.
If you put it in a single strap bag like a satchel then it can dig into your shoulder after a few hours. If you use a briefcase then you have to mount it somewhere or continually have one hand occupied.
