Also, Vue keeps in memory the last operations for you to undo them. If the purge occurred automatically, this option of undoing would be lost. Photoshop has pretty much the same options, and I think it makes sense.
Dunno if you noticed, but with the last update, voluminous data such as displacement & SSS can either be kept in memory or flushed after a render completes. So this option works with purge memory. If this was your final render, you may want to flush the data, or keep it in memory to render again. Same if you don't have enough memory, flushing the data will help you work better on the scene, but it will have to be computed again for next render. I think this can help a lot "older systems" to cope with the high memory and system resources needs of the new functions of Vue6.