24bit pixel plotting

Im reading the book "Tricks of the Windows Game Programming Gurus." I just now read over plotting pixels in 16bit, 24bit, 32bit colors. I know how to do them now but I dont exactly understand how 24 bit is working(although I do know how...but I hate not understanding how its doing it). Heres the code the book shows...

Code:

inline void Plot_Pixel_24(int x, int y, int red, int green, int blue, UCHAR *video_buffer, int lpitch) { // this function plots a pixel in 24-bit color mode // assuming that the caller already locked the surface // and is sending a pointer and byte pitch to it // in byte or 8-bit math the proper address is: 3*x + y*lpitch // this is the address of the low order byte which is the Blue channel // since the data is in RGB order DWORD pixel_addr = (x+x+x) + y*lpitch; // write the data, first blue video_buffer[pixel_addr] = blue; // now red video_buffer[pixel_addr+1] = green; // finally green video_buffer[pixel_addr+2] = red; } // end Plot_Pixel_24

When it plots the three different colors is it just ploting them very close to each other to where it mixes the colors or what? I was also wandering why [pixel_addr] has to be blue, [pixel_addr+1] has to be green, [pixel_addr+2] has to be red.

video_buffer is a pointer to surface and lpitch is a pointer to the direct draw surface description lpitch.