There are two kinds of syringes all fountain pen users should own. I bought these from Goulet years ago:
The bulk syringe makes it way faster to clean a cartridge converter fountain pen. You just fill it with water, stick it into your nib section and flush like 50 cartridges of inks through it in seconds.
The ink syringe on the other hand can be used to different things. Some use them to re-fill used cartridges, I use them to fill my converters all to the top. And can also be useful for quickly rinse converters.
They are not expensive, but can make your fountain pen life much more enjoyable. Makes it way faster to clean or fill your pens.
If you use fountain pens, I think you should have them. Made my life so much easier.