ESM (Enterprise Security Manager) uses "message-based" policies to collect and evaluate data. So, many legacy customers may have ESM in place, and may want to continue using those policies.
In that case, we must migrate those policies and they become "message based" standards in Standards Manager. You must have a mechanism (agent) to collect message-based data, as it differs from "raw" (bv-control uses "raw", as does the new CCS v11 agentless data collection technology). Both methods collect data from the asset, they just do it differently.
In short: if you're not migrating from ESM, there's no (good) reason to enable mesage-based data collection in your environment. Some may disagree with me, but I firmly believe RAW > Message... your mileage may vary.