Writing well in Spanish is not as common as you might think. When I began graduate studies in Spanish, I was so nervous that I arrived thirty minutes early for my first class. Several other people were already there, conversing in rapid Spanish. I was terrified. What had made me think I could hold my own with these people? However, I stayed and came to realize that many people who speak Spanish fluently have learned as a child in a Spanish-speaking family. That does not mean they can read it easily or know the grammar or can write. If you take some care, then your written Spanish will be as good as that of many native speakers.
What possible occasion would you have to write? You may not even write much in English anymore. But there is an argument to be made for the fun of writing in Spanish.
Along with speaking, writing is one of the active skills, where you are producing Spanish rather than absorbing it. But it’s different from speaking. With speaking, listening, and reading, you should let go and immerse yourself in the present reality. You should strive for communication and the experience of the moment. You should try to associate Spanish words directly with objects and actions and situations. You should avoid translating.
It’s different with writing. Writing in Spanish is the best way to polish your rough edges. It is the time to consider correctness. When you write, you can look up words in the dictionary, consult a verb book or grammar notes, and carefully construct a correct sentence.
Not only will you be thinking in Spanish but you’ll be thinking about Spanish. This is the only time it’s helpful to try to analyze the language.