Do I need a hydrating serum under my moisturizer?

No. A well-suited moisturizer can provide enough hydration and help reduce water loss on its own. If the moisturizer already keeps the skin comfortable, adding a serum simply because it is popular may not add much value.

A hydrating serum is optional

A serum is optional if it adds comfort or meets a specific need. I prefer adding a step for a clear reason, not because every routine online includes one. More layers do not automatically mean better hydration.