Regardless of what level a teacher teaches at you are always looking or materials and activities to do in your class. It is always a challenge to have new ideas and activities to support and help students because the world changes. This leads to an constant need to remove old inefficient activities and bring in new fresh one. The primary purpose of activities is to provide practical application of the skills taught and experience in school.
For the math teacher you can naturally make your own math problems. However, this can quickly become quietly. One solution to this is to employed other worksheets that provide growth opportunities for the students with stressing out the teacher.
There are many great websites for this. For example, education.com provides many different types of worksheets to help students. They have some great simple math worksheets like the ones below
There are many more resources available at education.com as well as other sites. There is no purpose or benefit to reinventing the wheel. The incorporation of the assignments of others is a great way to expand the resources you have available without the stress of developing them yourself.