A part-time freelancer with limited expendable hours will find this dual use of time efficient and rewarding. You write on your own time, at your own pace. Assigned articles inevitably mean deadlines. A few writers simply cannot function under deadline pressure. Others have unpredictable schedules that include intrusive jobs, children, health emergencies, and other uncontrollable elements.
Spec writing allows one to write regularly, but to keep a flexible schedule.
Although I try to write daily, I find my output is lightest when my five children are at home. During school vacation periods and summer breaks, I adjust writing tasks to allow maximum attention to both my family and my career. Some writing projects lend themselves best to spec writing. Personal essays, humor, fiction, and poetry are best produced, then submitted on spec. Brief pieces, like fillers and anecdotes, fall into this category as well, since their brevity and meager financial return don't warrant a flurry of time-consuming, costly letters for either writer or editor.