Folding ski poles are the best choice when packed size is your top priority. They collapse into sections connected by an internal cord—similar to how an avalanche probe works—making them small enough to stash inside or attach to a backpack. This makes them ideal for ski mountaineering and steep backcountry ascents where you frequently take your poles off and store them on your pack.
Compared to telescopic poles, folding poles pack down smaller and are typically a bit lighter, though they don't offer length adjustment for switching between uphill and downhill modes. Compared to fixed poles, they're more packable but slightly heavier and less bombproof due to the internal cord that can stretch or wear out over time.
If you're primarily doing resort skiing, fixed poles are the simpler, more durable choice. But if your adventures regularly involve stowing poles on your pack for technical climbs or travel, folding poles are worth the investment.