Condition block is the tool you would certainly want to use whenever you need to send your subscribers down different paths depending on some requirements.
To add a Condition to your flow, double-click anywhere in the Flow Builder and choose '+ Condition' as shown in the screenshot below.
Just choose a condition and then choose different outcomes for customers who match these conditions and for those who do not match them.
For example, you can add different tags to your subscribers and then depending on these tags send them one message or another.
Or you have several sequences you want your visitors to see one after another. You can check if the customer is subscribed to the first one, and, if yes, subscribe him to the second one:
It is also possible to use System Field or even Custom User Fields information as a condition. For example, you have a quiz only available for women:
You can even use several conditions in one block and make your subs go down the flow if he/she meets all of them or any of them.
All in all, condition block is one of the most versatile tool as it lets you create pretty complex bots, so feel free to experiment and see what you'll be able to create!