I've done it for a couple of separate reasons
>Been arachnophobic since childhood
>want to get into DnD
>monster manuals got illustrations of spiders and bugs everywhere
>forever GM in my group
>have to have another GM or one of my players go through the MM for me because too scared to see spiders
and then the other time
>years ago, playing something with too much math, probably gurps or some shit
>really good at worldbuilding and writing and general fluff
>can handle most math but this shit escapes me
>bring in one of my DnD buds to co-GM
>he doesnt mind since he's rainman levels of autistic math prodigy
>I do fluff, he does crunch, everything goes well