When the DC line is not properly decoupled or filtered, RF signals have a chance to inject RF power into other parts of the system where you don't want them. This often leads to a serious performance degradation. Therefore, capacitors are a fundamental building block in RF and microwave systems. In good condition they block direct current and allow alternating current to pass through. This is why they are often used to create filters, protect against DC and create bypass networks. For more info: https://www.heynen.com/products/knowles-dli-v-series-single-layer-capacitors/