but I don’t know what it actually improves

Depends on what you’re upgrading from.

I don’t have this extruder, I’ve heard it’s ok but I can’t vouch for it personally. However I can tell you how it differes from a stock Ender 3/pro/v2 if that’s what you’re coming form. To avoid confusion it might be worth mentioning that the word “extruder” is a bit ambiguous in 3d printing, but when talking about printer hardware most people use it to only mean the motor and drive gears for the filament.

The stock Ender 3, and many other printers, have bowden drive which means that the extruder is mounted on the printer’s frame, and the extruder pushes the filament into a tube leading to the hotend. This kit changes the printer to direct drive, where the extruder is mounted right on top of the hotend. This can reduce print artifacts since it shortens the filament path, but printer firmwares also have features to compensate for this (pressure advance in Klipper and linear advance in Marlin IIRC). The disadvantage with direct drive is that it adds weight to the toolhead, which can cause ringing (a.k.a ghosting) artifacts at higher print speeds. However the Sprite has a geared extruder so it uses a smaller and lighter stepper motor so I don’t think that’s anything to worry about. Klipper firmware can compensate for toolhead weight (resonance compensation) but I don’t know if Marlin has got that yet. Either way, having a direct drive extruder will allow you to print flexible filaments.

Speaking of geared extruders… When melting filament and pushing it through the nozzle, the amount of force needed is going to increase the faster you try to do it. When the required force exceeds what your printer can deliver then it’s going to start underextruding. If you have this issue you can see that printed lines will look thin in the middle, and thicker where the nozzle changes direction and the printer moves more slowly. This problem can be fixed by printing slower if you don’t want to upgrade the printer. An extruder with a gear ratio will be stronger, so it can be more consistent with the amount of extruded filament.

The stock Ender 3 etc. have a hotend where the bowden tube, which is made of PTFE a.k.a. teflon, goes all the way down to the melt zone. This slightly reduces the risk of clogs, but it also means that you never should print above 220C because the teflon degrades at higher temps and begins to release toxic gases. This kit appears to have an all metal hotend, so the PTFE tube is kept away from the hot parts of the hotend, which is why the product description says you can print up to 300C.

As for this particular hotend/extruder kit, I think the main advantage is that it’s a very easy and convenient upgrade. I think the quality is ok, but if you like the idea of modding there are plenty of other options. If you buy parts (hotend, extruder, fan, light weight stepper motor) from well respected manufacturers and print the carriage and fan duct yourself I think it’s possible to get a solution that is better in every way (lighter, stronger extruder, more efficient hotend, more cooling) for about the same amount of money. However I don’t think you’ll really notice much difference compared to this kit, and I don’t think it’s worth it unless you really enjoy the modding process. If you buy a Bondtech high flow nozzle you’ll probably get really good performance from the Sprite.