Having worked in the intel community (long ago), I suspect the practical purpose of Sentient is to replicate the mind of the intel analyst -- to select and fuse multiple intelligence sources into a coherent story that suggests an underlying human activity that would be of interest to investigate further or classify as actionable and then pass it along to an operational group like law enforcement or the military.

Another possibility, of course, is FUD. Promote a project as being more impactful than it really is so that your political critics are distracted by it and thus overlook your other projects that are more important, short-term, and real.

You do have to wonder. Why would a secrecy-driven org like NRO employ a meaningful and daunting project name like "Sentient" unless you want outsiders to become take an interest in the program? When you want a program to fly under the public radar, you name it something meaningless and innocuous like "BranMuffin" or "Spatula", not meaningful and fearful like "KillerFlyingRobots".