I think you are probably having it done to identify metabolic defects, perhaps an energy deficiency caused by a problem in the mitochondria (these are the energy factories of the cell).
I am looking to see if I can have one done since I have some obvious problems with my mitochondria. As far as I know, it is done by a needle biopsy and looked at by electron microscopy.
If you would like to get a biopsy to check for mitochonrial problems, my understanding is that they will show genetic or inherited problems but not the type that ME patients have -- which are usually acquired. For that you would do better with something like the Acumen Labs testing in the UK.