Task migrations are (relatively) expensive, and runtime-per-core lets you write request handlers that aren't Send+Sync. It's the kind of thing that looks good in benchmarks.

I wouldn't want to rely on it, because you can't rebalance uneven loads between cores, and a single blocking task would block all requests assigned to that core.

In practice though I haven't seen much of a difference. Actix and Axum are basically the same.