Hello Wenyu,

To optimize the reusability of your code is smart to seperate different logic/processes in different flow so you can re-use them in different places, for example you can create a new booking, but maybe there is also a situation where you want to edit a booking, in both cases you need the validation and book option but not the creation option.

This is why you often separate the validation flow from the creation flow. 

But maybe there also need to be send an email to update the customer about its reservation, sending an email is also a process that you would like to reuse for example.

Hope this helps,

Good luck!