'output vec4 fragOutput1' but 'RTTDepthTextureObject->Create2D( with 1 compnents float/uchar/ushort'...?

‘output vec4 fragOutput1’ but ‘RTTDepthTextureObject->Create2D( with 1 compnents float’…?
why create the depthTexture with 1 comp and FLOAT, but the shader can use ‘vec4’, and fill the rgb with depth