Projecting a return from a savings account is more straightforward - interest is paid at regular intervals, and the compounding effects of interest upon interest produce the final return (we are assuming you are not withdrawing amounts from the account, as this would make a difference). Interest rates do fluctuate over time (generally linked to the rate of inflation), so an average rate should be used.
Projecting the returns for Investment policies is quite different - the return from investment plans are linked to the underlying investments (such as Goverment Bonds, Blue Chip shares, shares in smaller companies, Overseas shares, specialist sectors, etc), and each will produce different returns at different times, and some will be more volatile than others (i.e. funds investing in smaller Japanese companies may return huge returns one year, and huge losses in the next year, whilst Government bonds may produce unspectacular, but steady returns), so an averaged return over time needs to be inputted. Whilst it is impossible to use a crystall ball to gaze into the future, an assumption needs to be made. See the specific help section for more guidance.