Traceroute was immensely helpful for me figuring out why my favorite video game tend to have large latency and massive lag spikes.

Geographically, where I lived, my connection should have been about 220 miles directly to Chicago. Instead, my connection traveled about 180 miles west to Minneapolis then 350 miles down to Chicago. Because this involved a bunch of extra network switches, my packets would often get buffered and sometimes delivered out of order (this was obvious by how the game worked).

A fiber provider came to town and solved all of my connection issues. Not only was the connection inherently faster, but it had fewer hops and was routed more directly to Chicago (where this game had a datacenter).

I think I went from nearly 100ms ping to 10ms ping.