T-Slot Aluminum Design: Slot Size

What Aluminum T-Slot Bar Should I Use? aluminum tslot 10PCS T Slot 2020 Aluminum Extrusion European Standard 300mm Length Anodized Extruded Aluminum for CNC DIY 3D Printer and Industrial Bracket Making aluminum t slot extrusions The piece is divided down the middle by an about 8mm slot On each side of that slot is an aluminum piece that is about 15mm wide So that from

aluminum t slot framing My question is - why use expensive extruded aluminum T-slot? I mean, for a CNC router it makes sense because you need sub-millimeter