P2417H, multiple monitor set up

@rizzla_  According to Dell's product page, the P2417H has DP, HDMI, and VGA inputs.  So if your laptop has DP and HDMI outputs, and you're already using HDMI for one of them, then all you'd need is a DP cable from your laptop's DP port to the second P2417H's DP input and you're good to go.  Note that there's full-size DP and Mini-DP, so make sure you get a cable with the correct connector on each end.  There are DP to DP, Mini-DP to Mini-DP, and Mini-DP to DP cables.

@rizzla_  an HDMI splitter would give you a mirrored image across all attached displays, not independent/extended displays.  HDMI does not support running multiple independent displays from a single output.  Only DisplayPort does that (and USB-C, which uses DisplayPort for video traffic), and it can be achieved via daisy chaining if you have displays that support it or else with an MST hub, which looks like a splitter but allows the attached displays to be treated independently.  A Thunderbolt 3 to Dual DP/HDMI adapter will also support this because Thunderbolt 3 can tap into two independent DisplayPort interfaces coming from the GPU.

From an image quality standpoint, all display connectors look identical except for VGA because all non-VGA display connectors send a digital signal.  So as long as the connector and cable you're using have the bandwidth capacity to run the setup you want to run, then they'll deliver the same result.  (Note: There are some special cases like HDR and G-Sync that can require you to use certain types of display output and input connectors, but none of those cases would come into play with the display you're asking about.)

The USB ports are essentially a USB hub built into the display.  So for example if you wanted to use an external keyboard, mouse, flash drive, webcam, etc. with your system, you could plug them into the display and then just have a single USB cable running from the display to your system, rather than having to plug everything directly into your system or purchase a separate USB hub to gain enough ports to connect everything.