When a non-moving GIF is useful
Some places require a GIF but shouldn’t move: email signatures, forum avatars with animation rules, print layouts, or thumbnails. Static versions also help accessibility — WCAG asks that moving content lasting more than five seconds can be paused, and offering a still alternative is a simple way to respect people who are sensitive to motion.
If you just need to stop a GIF temporarily to look at a frame, use Pause GIF, which plays, pauses and steps through any GIF without changing the file.
GIF or PNG for a still?
For most uses a still is better as PNG: identical pixels, broader editing support, and often a smaller file — see GIF to PNG. For icons, use GIF to ICO. Keep it a GIF only when the destination specifically demands the .gif format.