It's most likely for a slotting Cutter bit. Used for well cutting slots. Edit. (It's missing the actual blade that goes on it, you can buy them separately if you want)
https://www.amazon.com/dp/B07V2GYC1M/ref=cm_sw_r_cp_apa_glt_fabc_G4ZFYMG9KEB5X3712FC9
Yakamoz 1/4 Inch Shank 3-Wing Adjustable Slot Cutter Router Bit Set with Bearings | 6-Picecs Slotting Cutting Blades, 1/2 Inch Cutting Depth, 6 Different Cutting Widths https://www.amazon.com/dp/B07V2GYC1M/ref=cm_sw_r_awdo_navT_g_HDHRGVPHCQCS7G3TZK1A