The airdrops you are talking about seems not to be some official or legit airdrops, but if you find some than joining them will cost us no money but only time and we should join them if they are legit to gain some profit,  but as we know these are just graphical pictures with some value and some times with no value but they ask us to follow their twitter handles, which is almost free to them like they are gaining free followers by spending no money at the time,
So all of these airdrops could be a strategy to gain followers or email lists, well out of 100 airdrops some are real and there is only 1% chance that you will won it.