At the moment, very few casinos give you a sign up bonus just to play. The vast majority will give you a bonus for your first deposit. There are also sign up free rolls, but with such bonuses you always have to do (usually) quite a lot of wagering to be able to withdraw. In other words, the sign up bonus amount does not matter as you have little chance of a payout, so I advise you to use it as a bonus to test the casino.
That is correct, more on deposit bonus not the sign up bonus.
Because the sign up bonus can easily be abused by users, as you don't need to shell out money.
Usually, you will find this sign up bonus for a very new casino trying to attract players on their site.
Also, keep in mind that most bonuses have conditions that you need to meet, so check out those conditions first.
Only few gambling platforms offering with the signup bonuses afaik for the first account created in the winz.io they are offering with the free spins only and the other platform they are now offering the depends on the deposit they made they have their own free spins in a particular games only. But having free spins bonuses gives more chances to the player to get a good outcome still it depends on the players designated to platform to play with. For me deposit bonus, wager bonus and a number of wages is an ideal gambling platforms for active players.