Why? Because it saves money and you can use an empty syringe many times. You do not have to rely on cartridges from one brand only. Some manufacturers produce cartridges that can be used with only one type of fountain pens such as the Pilot Parallel Fountain pen.
Though I do not want to buy cartridges that cost almost 4 EUR like those for the Pilot Fountain pen so I can write with it...
Here's a video that shows how to refill your ink cartridge.
I have another video but I will upload it later. I hope you enjoyed this. Let me know if you are going to do the same. The usage is easy just beware of little children and put this away from them so that they cannot reach it.
Thanks for reading