A lot of it depends on your ideals I think.

There is environmental impact from chemicals such as pesticides and herbicides from farming cotton and the manufacturing process. For me personally that would the reason I chose organic cotton.

I also believe that other reasons people choose organic is there could be chemical residue on the fabric which could be absorbed by their body when they wear it and cause issues such as allergy or other reactions.

I'm sure if you googled pros and cons of organic cotton you'd find stacks of interesting articles and debate