Yes, flexibility. You don't need to have 2^24 hosts for a 24-bit address space to start becoming cramped. You have to divide up that space into subnets that roughly match your physical network structure, which depends on accurately anticipating future growth; otherwise you'll have to suffer the pain of renumbering later on.

The real benefit of a /48 or /32 IPv6 subnet isn't that you never run out of individual addresses. It's that you're practically guaranteed never to run out of identifiers at any level of the hierarchy.

... but you don't have 10.* to yourself.

I've seen many cases where companies were bought, merged, or linked to other companies and had to put complicated NATs in place to translate org A's 10.* addresses to/from org B's overlapping 10.* addresses.

Then you have the problem of connecting to a VPN for 10.10.0.0/16 from a coffee shop with 10.10.0.1 as its router.

My solution integrates in other networks while having its own private addressing. Think something like a VPN (it's not a VPN but that's a story for an other day). Even if I allocate a random subnet like "10.5.212.0/24" I still take the risk of being unlucky and conflicting with something. On top of that I then only have 256 individual IPs to play with which means that I need to have some kind of dynamic allocation scheme instead of just using the MAC address to have a globally unique ID. This is of course even worse if you want to allocate multiple subnets.

IPv6 is much simpler to deal with.

10.* is private. His IPv6-only project was probably using the Internet, so his computing farm at point A could talk with his high performance cluster at point B without a tunnel.