Agreed. Depends on rig design. Often with no forward lowers a baby stay is required to offset aft lower tension and/or prevent mast pumping. Also, even if not easily adjusted, it can still be used for mast prebend. Some rig designs call for this. One of the beauty's of a frac rig is that all of this can be taken care of by the back stay and fore stay. By the way. I like the boat in the above picture.If it is of fixed length (not easily adjustable underway) then it is there to stabilize the rig, keep the mast from pumping, or inverting. Typical of cruising boats.
If it is adjustable via blocks and/or on a track like this C&C, then it is for rig tuning. Hardening it bends the mast and flattens the mainsail. Racy.