What is Collet Chuck Who Needs One

3x12pcs er25 chuck collet 1/8 to 5/8 sp collet set machine tool holder er2 j2e2 this item is in the category business, office & industrial\cnc, metalworking & manufacturing\workholding & toolholding\workholding\collets\spring collets. 3X(Special slanted slots increase stiffness Durable and practical Protects tool holder from contaminants Use with each standard ER collet chucks for coolant-thru tool applications Colour silver Material sp steel Size 15 x 25mm Package Contents 12 x router collet).