Fixed length poles are one solid shaft with no moving parts, making them the simplest and most reliable option out there. They're lighter than adjustable poles, more durable, and offer better stiffness for power transfer when you're planting turns. For most resort skiers, fixed poles are the way to go.
The trade-off is that you can't change the length on the fly or pack them down small. If you're skiing backcountry and need shorter poles for uphill travel or longer poles for descents, adjustable or telescoping poles make more sense. Similarly, if you need poles that fit in a travel bag, fixed length won't work.
But if you spend your days at the resort, fixed poles are cheaper, lighter, and less likely to fail you mid-run. No locking mechanisms to freeze up or slip under pressure—just simple, dependable performance.