Most standard stock fountain pen nibs are designed to give a consistent, unvarying line. But some users, such as artists, calligraphers, or just anyone who wants to add more personality and distinctiveness to their writing, are attracted to fountain pens precisely for the ability to use customized nibs which provide for varying line-widths.
If you are happy with a consistent, unvarying line with your pen, then most likely you will not have any immediate need for a customization. But if you want to achieve line-width variation, you will most likely be considering a customization to Stub, Italic, or Oblique points, and possibly considering adding flex as well.
Other customizations are possible as well - regrinds to reduce the nib point to a size not provided by the manufacturer, and retips, to make a standard nib broader than its original size. Have questions? Feel free to contact us for more information.