This life insurance variant takes into account time value of money. Due to inflationary erosion of value, a pound today is not worth the same value in the future. To counter inflationary loss of value, the cover amount on an Increasing Term Life Insurance increases gradually to mitigate the loss of value over time.
The ideal type of increasing policy is index-linked, where the sum assured can be increased each year by the increase in the Retail Prices Index (RPI). This also means that your premiums increase each year.