Voiceflow has 2 types of variables: flow variable and project variable. If you use flow block with these variables, you need to know the difference between those.
BTW, if you use project variables only, you don't need this. However, that means all the variables are global and persistent. In that case, you will need to care about initialization of variables and if you forget it, sometimes it may cause what you don't expect.
May be some help!