“The moment is freedom. I couldn’t live by a rigid schedule. I try to live freely from moment to moment, letting things happen and adjusting to them.”-Bruce Lee
You have certainly experienced flow state yourself. Like most other states of consciousness, it does not require knowing what it is to experience it. While many spiritual peoples may have pursued flow state as a purpose in life, it does not require intention to experience. It is merely a state which happens when one is engaged in a task which is an appropriate level of challenge for their level of ability. One can stumble upon flow state or, as designers focus on, one can attempt to induce it in someone else.
To the designer, flow is important because it keeps the user feeling engaged with the product, which increases their usage. Flow state is also generally associated with positive feelings, and of course you want your users to have a positive association with your product. Tasks are made simple and lead directly into one another in a way that is intuitive and effortless for the user to navigate the site. Designing with flow state in mind means making a product that the user immediately understands but which also challenges them to explore more of the product. The ultimate purpose of this is to put the user into a state that they naturally want to experience in the first place, and minimize frustration. By doing so, you ensure that the user enjoys the experience overall. Unfortunately, the effectiveness of designing for flow state is not simply a matter of creating a delightful experience.
It may seem that the downsides revolve around loss of self-control, similar to any other addiction. That as long as a person can remember that they have needs in their life other than flow state, then the negative aspects fade away. But does a person have that ability when they do not even know they are chasing flow state in the first place? A drug addict knows that they are getting high. Does the average person on the internet know that they are, too?