Formal or academic writing and informal writing stand for two main styles of writing, the first being impersonal, dispassionate, serious, involving people we do not know well such as employers, a principle, the president, and the latter is the opposite, it is personal, it is more common when speaking, it addresses to friends through letters, emails, or text messages. Both styles are associated with choices of language, grammar, and vocabulary. It is my strong belief that the informal writing is indispensable and needed to gain formal writing abilities, therefore it is more important for several reasons.
Firstly, it is fundamental, vital for humanity. Every (educated) person can express himself or herself informally, by speaking or writing after a certain age. Experience gained informally helps people develop formal abilities over their course of existence. For example, a sophomore, with a wide-range knowledge of informal writing will develop over college proficient and critical writing qualifications.  For one to solve an equation, he must first learn how to count and understand how the algorithm works. For one to be able to express himself formally, he must first learn how to express himself informally.
Secondly, informal speaking and writing need less time, energy, and knowledge. Informal writing is simple and immediate, while in the formal experience, people tend to surf the internet or seek the advice of their superiors. Informal writing addresses a problem straight on the subject, while formal writing might be deceiving, time-wasting, people losing sight of their goal. When speaking, it is a query of respect whether one expresses himself formally or not.
Admittedly, formal writing/speaking is, for example, necessary when applying for a job or addressing a matter to a head of a state. Formality usage denotes admiration and appreciation towards a superior (an employer).
In conclusion, informal writing/speaking is the base of knowledge needed for advancing to gaining higher language abilities. Formalities are in a couple of instances necessary, therefore every man will at least once in his life need to address himself formally, by word of mouth or text.