I truly believe that boh Formative and Summative assesments are very important in a classroom. A Formative assesment will help the teacher determine if his/her students are understanding the content being taught to them. Depending on the outcome, the teacher will be able to decide if she needs to spend more time teaching that content (using a different teaching strategy) or if they are ready to move on. A Formative assesment doesn't have to be a quiz or test, it can simply be asking some questions, playing a kahoot game, an assignment; Formative assesments are not graded, they are only used to determine students undersranding on the content being taught. Summative assesments on the other hand, are taken for a grade.
A Summative assesment is given to the students at the end of a unit to determine their over all understanding of the unit. Summative assesments can be given to students in a form of a test, or a project. Based on the students results, teachers can adjust their teaching strategies for the upcoming units, and also for the upcoming school year.
Answering your second question, I would use a formative assesment more in my classroom, because based on the students results, I will be able to determine if they are ready to move on. I would use a Summative assesment less, because they are only given at the end of a unit, quarter, semester, and year.