What is the reason for certain yoga poses?

Yoga asanas (postures) start the process of opening up the
body in order for the prana (internal energy) to increase its
flow thru the meridians of the body. These meridians connect
with the glands and organs of the body and when flow of
prana is balanced charge the glands to produce the hormones
which run our bodies and act as catalysts for the optimum
operation of the organs. Some of the other purposes of asanas is to
develop flexability, strength relaxation of mind and body, to
find calmness within,seek stillness within and understand your
relationship with humanity.

Yoga can help you keep in shape,and it is really a good exercise.

A friend of mine who practices yoga said certain poses can help squeeze or stretch the area’s around certain organs thus releasing toxins that are sometimes trapped since we bombard our bodies with unnatural foods it’s easy for our system to overlook some of the toxins within our organs. I don’t know if any of this is true but that’s what a yoga-ist believes. Also stretching the muscles releases acids which is essentially ripping the muscle and it healing gets stronger in return.

If you are asking specifically about the “very difficult poses” they are not essential.
They are useful for exercising focus and concentration and as @pleiades mentions for specific release and healing – if you need that.
Unfortunately I see people pursuing extreme poses mostly out of ego, bragging rights, which is not useful.