An NFT airdrop can be initiated for several reasons. You may receive an NFT airdrop for holding a certain NFT in your wallet, promoting a brand in some way, participating in a giveaway, or even as a gift which is more a marketing strategy rather than a gift.
Just to be fair, NFT airdrops can be extremely valuable. A good example of a valuable NFT airdrop is the Bored Ape Mutant Serum. Holders of Bored Ape Yacht Club NFTs were airdropped variations of what's known as the Mutant Serum NFT.
Although these Mutant Serums were airdropped for free to holders, it didn’t take long for them to begin selling for over 3 ETH (thousands of dollars) on secondary NFT marketplaces. In fact, on January 2, 2022, a Mega Mutant Serum sold for 1,542.069 ETH ($5,907,542.97).
That being said, not all NFT airdrops are valuable. Some may be completely worthless, and some may even be scams. If you ever receive a random NFT in your wallet without your knowledge from an unknown source, you should be extremely cautious in how you handle the asset and try to avoid selling it or trading it in any way.