If this spoke were a straight-pull, then the flats would serve as a place where one could grip the spoke with more standard tools compared to spoke holding pliers:

Without something to hold the spoke you cannot adjust tension via the nipple as the spoke will generally turn with it.

Less of an issue with bladed spokes, but the same applies with j-bend spokes and issues with spoke torsion, this can be mostly mitigated via other techniques though.

If this spoke were a straight-pull, then the flats would serve as a place where one could grip the spoke with more standard tools compared to spoke holding pliers:

Without something to hold the spoke you cannot adjust tension via the nipple as the spoke will generally turn with it.

Less of an issue with bladed spokes, but the same applies with j-bend spokes and issues with spoke torsion, this can be mostly mitigated via other techniques though.

Update: Looks like they're J-bend. My guess is that the feature may be there to assist with machine building, but my original answer remains intact as it's all still relevant info to someone else who happens across this topic :)

If this spoke were a straight-pull, then the flats would serve as a place where one could grip the spoke with more standard tools compared to spoke holding pliers:

Without something to hold the spoke you cannot adjust tension via the nipple as the spoke will generally turn with it.

Same applies with j-bend spokes and issues with spoke torsion, but that can be mostly mitigated via other techniques.

If this spoke were a straight-pull, then the flats would serve as a place where one could grip the spoke with more standard tools compared to spoke holding pliers:

Without something to hold the spoke you cannot adjust tension via the nipple as the spoke will generally turn with it.

Less of an issue with bladed spokes, but the same applies with j-bend spokes and issues with spoke torsion, this can be mostly mitigated via other techniques though.