This sentence is advising someone to not act too wild or careless because it won't bring any practical benefits. The use of "放肆" conveys a sense of being excessively unrestrained, while "没什么用" indicates that such behavior has no practical use or purpose.
In life, it's important to strike a balance between expressing oneself freely and being mindful of the consequences. Being too reckless can often lead to negative outcomes, causing harm to oneself or others. It's essential to consider the potential ramifications of our actions before acting impulsively.
Sometimes, people may feel tempted to let loose and be carefree, as it can provide a brief respite from the pressures of life. However, it is crucial to remember that moderation is key. By making informed decisions and weighing the consequences, we can avoid unnecessary troubles and focus on achieving meaningful goals.