Primarily for different kinds of dress, in my case. In hot weather, particularly when I wear shorts (sorry for THAT image), I don't really have a place to hide a full-sized pistol. In that case, I can pocket carry. In cooler weather, especially when I can wear a jacket or coat, concealing a full-sized pistol is much easier.
There's also an argument (the validity of which I will leave to those who really know more about gunfights than I) that since bad guys will also wear more layers of clothing in winter, it's a good idea to carry something with more oomph in the winter than in the summer.
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.