Just writing does not always make you improve. You need good feedbacks indicating your mistakes, weakness, and the way you fix these.
However, people who are familiar with English sometimes say they've practiced their English by translating all phrases they got in their mind in everyday life. Writing on your own have some certain amount of learning effect.
In my experience, using English (and getting feedbacks from it) in various situations is important to get solid skill. Different situations need different representations.