What does the Windchaser Departure pod do?

The difference is cosmetic. Your Aniimo shows flower petals where the sparkles would be.

The pod functions identically to a Sparkling Cube, so you use it in the same places: thrown at a wild Aniimo to catch it, or applied to an egg in the Hatchinator before it hatches. What you get is the season-exclusive legendary appearance, which shows flowers instead of the usual sparkle.

Because the petal effect is season-exclusive, use the pod on an Aniimo you actually want wearing that look. A regular Sparkling Cube gives you the standard sparkle.

Not settled: This pod behaves like a Sparkling Cube, so it has the same open questions. It is not settled whether every Aniimo it touches is guaranteed to come out sparkling, or whether its stats are guaranteed perfect.