Feature updates are biggies which require a complete reinstallation of the operating system. Issued twice yearly they are in effect a new version of the operating system, hence the need for a full reinstall. You can defer feature updates for up to 18 months if you have a pro version of W10, the hope in doing so being that any bugs will be sorted by the time you take the plunge. Best to do a full back before installling a feature update.
Not to be confused with the smaller and more regular quality/cumulative updates which tend to be issued monthly and cover general maintenance.