The best reason to write out your lines before trying to use the correct number of syllables is:
It is easier to change words once you have your ideas down.
By first getting your thoughts on paper, you can focus on the essence of what you want to express. Once the ideas are clear, you can then refine the lines to fit the desired syllable count, making it a more fluid process. This approach avoids the frustration of trying to mold your thoughts to fit a rigid structure before fully articulating them.