The familiar Parker-style ballpoint refill is an ISO standard, often referred to as G2 (not to be confused with the Pilot G2 gel pen, which uses quite different refills). A lot of pens from many different manufacturers use these refills, and a lot of different brands make their own refills in this shape. That means there can be quite an advantage to using a pen that takes them - they're readily available when you need one, and there are lots of choices.
Ballpoint ink, gel ink and hybrid ink; pressurised 'space pen' refills; and a good selection of tip sizes and colours, all are available in the G2 'Parker-style' format. Parker pens obviously use them, but so do many others. So if you like the idea of having a lot of options for refills, you can pick from this huge selection of pens.
