WD19 without power supply

@Seb15  The WD19 180W comes with a 180W power supply.  When the WD19 is using a 180W power supply, it can pass up to 130W through to the attached system.  Assuming you're looking at the current XPS 15 9500 model, it is designed for a 130W power source if you order a configuration that includes an NVIDIA GPU.  If you order a configuration that only has an Intel GPU, it is designed for a 90W power source, in which case technically you could get the WD19 130W instead, which comes with a 130W power supply and can therefore only pass 90W through to the attached system.  The reason the passthrough amount is lower is because the dock reserves some power supply capacity to run its own internal electronics and provide power to any USB peripherals you might attach.

In terms of whether the WD19 is a good choice, if you'll have an XPS 15 with an NVIDIA GPU, then the WD19 is one of your ONLY good choices because the 130W that it requires is actually above the 100W max of the official USB PD spec.  Dell did something proprietary on some of their docks and systems to stretch the spec to carry 130W over USB-C, but you won't find that implemented on other docks.  Not even all Dell docks support that.  If you use an XPS 15 that is designed for 130W from a power source that offers less than that, you will either need to accept reduced battery charging speed and performance, or else keep the system's own power supply connected as well.

The only other dock you might want to consider instead would be the WD19TB, which is a Thunderbolt 3 dock, compared to the WD19 that only uses regular USB-C.  The WD19TB only comes with a 180W power supply, and the main reason you might want to consider it would be the display setup you plan to run.  The WD19 when paired with the XPS 15 9500 can run dual displays up to 1920x1200 each or a single QHD display (or a single 4K display but only at 30 Hz rather than the standard 60 Hz).  The WD19TB can tap into 4x more video bandwidth than the regular WD19 when paired with the XPS 15, and therefore it can run dual displays up to 4K 60 Hz each or triple displays up to QHD each.

The WD19 and WD19TB cannot be used passively.  You also will not be able to use the XPS 15's own power supply with them, because the XPS 15's power supply uses a USB-C connector.  The docks use Dell's traditional 7.4 mm barrel style connector.  And again, in order to supply the amount of power the system needs, the dock itself needs a higher wattage power source in order to cover the power requirements of the dock itself.  So if your XPS 15 was designed for 130W and therefore came with a 130W power supply, even if you COULD connect that to the dock, you wouldn't be able to get 130W through to your system.