There is this thing about shopping and airlines that when you check and forget, they send you bonus or discount for your possible shopping/trip. Same holds true for casinos, if you just register and leave they will send you some bonus to get you started playing.
This is true in some casinos. Even in other platforms and applications, it happen, which is why some are making this strategy to generate bonus from whichever they signed up to. Other people here in our country called it "farming" because you just make an account that somehow equates to planting a seed, leave it over a certain period of time, and wait for it to have bonus that equates to bearing a fruit. Many people do this here in specific applications.
Although just don't let your hopes up that high because while it works to some, it doesn't guarantee that it will work on you too. After all, not every casino will be generous enough to give a bonus to inactive players. Keep your expectations low because not every single time, you'll get rewarded from doing this. It's not a great way to enjoy gambling because you have to keep waiting before you can enjoy, without really a guarantee, but you could use some of the free bonus to indulge if ever they will really give any.