Cosmetic rewards change appearance, presentation or identity. Gameplay rewards affect what a player can do or how progression works. Individual offers can mix both, so a striking image does not tell you the full contents.
What the distinction means
A costume may be worth considering because you like it, without needing to justify it as a strategic improvement. Conversely, a resource pack should be assessed by the resources and conditions rather than the artwork around the offer.
What to consider before playing
Read the item description and separate appearance from function. If a purchase matters because you expect an advantage, look for an explicit explanation of that effect. If it matters because you want a particular look, verify the character or edition it applies to. Clear expectations are more useful than assuming every rare-looking reward is mechanically powerful.
The useful takeaway
Cosmetic rewards change appearance, presentation or identity. Read the item description and separate appearance from function.