A JPG is handy when you need a still from a GIF as a thumbnail, a profile picture or an attachment for a form that only takes JPG. JPG has no animation and no transparency, so the result is a single opaque picture.
JPG has no transparency, so see-through areas of a GIF are filled with a solid colour, white by default; you can change it under Options. JPG holds a single still image, so an animated GIF does not stay animated. The defaults suit most files. Under Options you can set the quality, aim for a target file size and resize the result.
If the GIF is animated, only its first frame is converted and the result is a still picture.