The motivation behind nearly all writing is to communicate. With the end goal to communicate well, you should consider both what you want to communicate, and to whom you plan to communicate it. This is no less valid for mathematical writing than for any other type of writing. The primary goal of mathematical writing is to assert, utilizing carefully developed logical conclusions, reality of a mathematical statement. Careful mathematical readers don't assume that your work is all around established; they should be persuaded. This is your first goal in mathematical writing.