To be complicated is to be difficult to solve, unravel, and/or understand. To be simple is to be clear, unraveled, and easily understood. Because communicating ideas is fundamental for design, simplicity becomes a necessary tool for doing so.
Simplification improves the efficiency of communication. This idea is exemplified in the process of simplifying fractions. 225/300 is equal to 3/4 but by replacing the former with the latter, the fraction is easier understood. By removing the unnecessary, you emphasize, highlight, and clarify what's left.
Read more: How to Communicate Your Idea