They are not necessary for the game, and can be done without.
In this case, the motivation to start doing the "optional" things like the bonus stage is not there.
But I thought, the start is the hard part, lets get started and things will get better and easier :).
I found motivation in it especially that the bonus stage has been constantly in my mind, and after many designs, I think I found a good and hopefully great one for Shadow Gangs.
There will be 3 types of Bonus stages, the best one and the harder to implement and the one which I am working on now will be in first person camera style, another one will be a side scrolling bonus stage, timer based, and the last will be a slot machine style bonus stage.