All of RFC1918 is legacy, but the idea that people need a choice of right-sized private nets is something of a legacy of classful allocations. CIDR variable sized nets allowed less wasteful allocation by 1993. A few short years later in 1996 private nets came along, so presumably the authors still had in mind the size categories of classes A B and C.
Another historical note, the Operational Considerations of RFC1918. Twenty five years ago, it was not a given that network hardware could handle the large change in address plan proposed. Picking a 1918 net of the same size as your internet assignment from the early 1990s gave people a familiar sized allocation.
The choice of which net in 1918 space is arbitrary. My home router is currently using a couple /24s out of 10/8, but no that was not the default. Consumer gear may have picked smaller and larger organizations larger, and from then become the conventions people expected. No technical reason.
The modern solution, IPv6, makes obsolete the idea of counting IP addresses, and provides relief to overlapping address space.
Go here and generate a ULA /48 for private use. There are more of these than IPv4 addresses, so there is a reasonable chance it will be unique to you. And each has 64 thousand /64 networks.
For internet, demand your ISP delegate at least one /48, possibly more if you can justify the address plan. It is possible to have all internet routable again, something like the simpler times prior to 1918 private nets.
