In this case, why encode the string instead of just having the options as plain text parameters?

Nesting, mostly (having used that trick a lot, though I usually sign that record if originating from server).

I've almost entirely moved to Rust/WASM for browser logic, and I just use serde crate to produce compact representation of the record, but I've seen protobufs used as well.

Otherwise you end up with parsing monsters like ?actions[3].replay__timestamp[0]=0.444 vs {"actions": [,,,{"replay":{"timestamp":[0.444, 0.888]}]}