Where did you come from ? You are sadly lacking in computer knowledge to even ask that question.
Memory sticks fit into memory slots. The original discussion was why or how do you plug memory sticks into a motherboard. And I answered it exactly why you plug memory sticks (or RAM chips or DRAM cards or whatever you want to call them) into a motherboard a certain way, and the disadvantages of not doing it correctly.
Memory sticks contain RAM mounted on them !
Not to put too fine a point onto it I suggest you do a technical course 101 on computers.

It is called 'interleaving'. While one slot is being read another slot is being set up. They swap back and forth. Since the functions overlap slightly you end up with a speed gain. The swapping is controlled by a memory controller. Most motherboards use two interleaving channels. Motherboards with single slots probably have interleaving capabilities built in but just can't use it. My motherboard has 3 channels of interleaving so presumably gains even more speed.
You can safely ignore interleaving if the board will still work but just remember that you lose some performance.
If you look at a motherboards specs you will see that (assuming say 4 slots) there is a preferred way the memory should be added. There is a reason for this. Memory should be preferably the same kind and speed to maximise the potential speed gains of the interleaving. If you find that somebody like Dell (only an example because they all do it) has used a single 16GB chip instead of the 2 x 8GB that they should have then realize that it is because a single chip costs fractionally less than 2 chips. They are maximizing cost savings to themselves as they don't care about you gaining a slight performance increase. By reading the above comments it seems that most of you don't understand or care that you have been ripped off.

I don't really know exactly what is the reason but I followed the instructions from HP
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/http://h20331.www2.hp.com/Hpsub/downloads/HP-Z600-Workstation-memory-configuration-and-optimization.pdf
My computer has 6 slots and 2 CPUs. It came with 12GB with 2GB in each slot. I put 6 8GB, one 8GB in each slot.
It appears that each CPU has 3 channels of memory and the channel 1 must be filled first. They said it's best to fill all 3 channels because the CPU then operates with 3 channels of memory. Each CPU has 3 slots and if you only have 1 CPU only 3 slots can be used.