It could be easier for load balancing or if you want to shard. You could do it at the DNS level at that point, without needing to strip the TLS bits, decode the HTTP request and go from there.

For the end user though I can't see much of a benefit of a subdomain over a URL path, or the other way around. I don't think it matters much but the author might have bigger plans in which having the subdomain could be easier.