To be honest 90% "it's fun to mess with". I had written my own looking glass type software with some historical statistics. I'm sure you could get similar data elsewhere but it was more about hooking into the route daemon updates and seeing general things or even watching what I was doing on other networks propagate live.

10% is having my own looking glass was significantly faster and easier for me to use. But I probably wouldn't have done it all just for that, it was just fun to mess with.

It's useful if you multi home.

Its not necessary for multihoming. I am el-cheapo multihomed using Linux router. I use policy routing with 2 default routes (primary and secondary) and everything works like a charm. I could even try to do some ECMP but I care more about stability of ping/jitter that raw bandwidth. And you dont need to bother to ask ISP for BGP or static IPs. You just take whatever you have and slap it together.

Yes, which is why I said useful. With a full table from two ISPs you know which ISP have a shorter path to a CIDR. It's also likely the same path where traffic also comes in from that CIDR.