The numbers are an indicator of market share and a leading indicator of advertising revenues. The numbers provided need to be interpreted with a little care as the national aggregates do not provide a full picture. The value of an audience also depends on what regions it is drawn from (the London market is particularly valuable, for example) and the composition of the audience (usually measured by social grade).
The radio broadcasters disclose some RAJAR numbers in their own statements (usually on the day on which RAJAR publishes the results of its quarterly survey). It is worth looking at the full numbers as this allows easy comparison. This also provides fuller information than broadcasters own statements which often quote selectively and incompletely from the RAJAR numbers.