Monetary values were removed from some British postal stamps in 1989 in order to minimize disruption to individuals and retailers when postal rates changed. Such stamps always provide the same level of service, such as first-class post, regardless of any future price increase, according to GBStamps.com.
The introduction of Non Value Indicator (NVI) stamps came about as a response to fast-changing postal rates. The switch to NVI stamps prevents retailers and individuals from potentially being left with obsolete stock following each new rise in price. The first NVI stamps were used for first- and second-class domestic mail. The Royal Mail has since introduced additional NVI variants for postcards, European and worldwide post.