Often times, a part may be necessary but is overly complex, and a simple representative shape would be sufficient for simulation. In other instances, there are recurring parts or a group of bodies or components that could be simplified into a single part.
When a part is complex, it is possible to use the Remove Features command, but that command requires time for all the calculations to properly heal the part. Replace with Primitives can quickly place a simple shape and remove (commonly known as suppress) the undesired part without requiring all the calculation time.
Other instances, when a group of parts could all be replaced with a single simple shape, Replace with Primitives comes to the rescue. The initial shape created using replace with primitives is a bounding simple volume surrounding all the selected bodies or components.
For example, an LED headlight may have numerous small components making up the configuration of the LED's. To run a thermal analysis, all the LEDs could be replaced with a single body, and the collective heat load applied to the one body. A more accurate thermal profile may be required to show the heat on each LED. Making each LED into a primitive more appropriately distributes the heat generation while keeping the element count down. The complex geometry of an LED isn't necessary when trying to get a fast look at the system-level thermal profile. Either of these methods can be valuable and time-saving when setting up an analysis.