I am thinking what is the importance of airdrop?
Airdrop is made for publicity.
What makes airdrop happened?
Of course the Dev's or the team that created the airdrop and all their supporter.
what is driving it continue to give away?
To attract more investor to continue their project of course it is all about business.
Is that only for promoting certain currencies?
No they could also promote it through bounty campaign or any other type of advertisement like posting it on some website but I think Airdrop is the cheapest way to promote the project.
should the airdrop continues happening
For me yes it is a good one for both the team and the participants,
Perks for the team low cost advertisement and people would surely talk and share about it.
Perks on participants free MONEY.