For some men? Just getting rid of the damned thing.
A particularly reasoned note in the comments section of the above link, by the post’s author Sean Philips:
…here are a few explanations I have heard over the years (none from this guy in particular).
-Wanting to make the ultimate commitment to being a gay bottom.
-Feeling they do not deserve sexual pleasure.
-A “simpler” means of achieving a more feminine form when compared to the cost and screening required for a proper sex change operation.
-Some men feel that without their penis their other parts become more sexually alive.
I am sure there are several other explanations as well if you were to ask around some. I find these motivations fascinating myself.
For further reference, investigate the terms Nullo or nullification in this context.