By flows we mean collection of Netflow, IPFix, ... generated by network components (routers, firewalls, ...).
The benefit of collecting flows is to bring network visibility in the ESM console about network usage, up to the Application-level (ISO Layer 7) in the ESM console.
So, while flows are collected, you should be able to see details on protocols used, applications, ... (depending on type of flows collected, some of them provides more details than others).
Another advantage of it, is to enrich detection capabilities of your ESM. Indeed, flow data is also available for correlation and you are able to define use cases mixing events with flows.
Example: an IPS alert indicating a possible exploit is raised to the ESM and collected flows in the ESM shows an unusual network usage for this particular server (volume of data exchanged, number of requests, ...)
Hope this clarifies.