While sometimes they add new features can be needed, often it is not needed (e.g. too fancy graphics), or can be done with other stuff you don't need to change this program (e.g. compression in tar; you can already use like "tar | gzip" to make a compressed tape archive file; actually I never use the compression options (or half of the other options that they don't need and are better done with other programs) in tar and instead use it separately like that). Sometimes also some thing can be removed, but sometimes they remove important stuff and include bad stuff instead.
Web browsers are another thing. Maybe the way to write a good one is to fork NetSurf and modify it as appropriate, and occasionally merge changes when they are appropriate to do.