Since the copper layer obtained by electroless copper deposition is extremely thin, it has high resistance and insufficient mechanical strength. It can easily break when exposed to heat or physical impact.
 
The purpose of electroplating is to thicken this copper layer by several to dozens of micrometers, such as 5-25 micrometers or even thicker. This process strengthens the hole walls, making them robust and durable, thereby forming reliable vias that ensure stable signal transmission between different layers.