For larger systems, that have many canisters, it may be impractical to have every single index on individual canisters. However if a system has key values they want to put a high-speed index on, this might make the system far more efficient for search of these key values. An example for this would be a hit-level log ID (TLTHID) or session ID (TLTSID).

Indexes for these might be centralized in the database server or the central reporting server, rather than on each individual canister, and can contain the location of the actual data. The advantage would be the Search Service can hit one location rather than engaging the entire environment, raking all the canisters for the results.

Yes, it would take a bit of extra space in the DB, but the advantage is to have much faster to find session/hit identifying values much faster with less system load. If it was in the database, the table could be copied into a in-memory table (with disk backup) so it is even faster to access.