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Why do do HDMI to DisplayPort adapters come with a USB charging cable?

I’ve been looking for an adapter that will allow me to use my Samsung monitor with my macbook. I bought a docking station with two HDMI ports, while the monitor uses DisplayPort. Because I also use this monitor with a Windows laptop, I already have a DisplayPort to USB-c adapter.

With my setup, I have 1 available USB-c port, hence buying the docking station. My plan was to buy an HDMI to DisplayPort adapter that would allow me to quickly switch between docking stations based on the laptop I’m using.

However, it’s been impossible to find an adapter that isn’t DisplayPort to HDMI. The ones I have found come with a USB charging cable. I’ve seen listings say that the cable needs to be connected to a power supply. Why is that? If Display Port to HDMI (Male to Female) is possible without charging, what makes the opposite not possible?

(I’m unable to paste the pictures I had)