It's basically: query the database for the list of nodes, then tell the LB which of those nodes you want to send traffic to. Something you come up with in 5min if you ever need it.
Debugging I guess? If you have more logic in your LB/routing, you may want to process all of it as usual, but skip the random selection part and talk to a specific server for a single request. Or it could be used for pull-style monitoring over the same LB as traffic (GET /stats/i-0001234). Or some weird cluster selection... like HP uses numbers in the domain, for example www8.hp.com
It's basically: query the database for the list of nodes, then tell the LB which of those nodes you want to send traffic to. Something you come up with in 5min if you ever need it.