RIM weighting is used when you wish to provide weighting for more than one variable to achieve an even distribution of results across an entire dataset.
It can also be used to produce an analysis in which the proportion of respondents in your sample is adjusted to match more closely to the proportion in the target population.
For example, if you wished to weight your samples so that they were 50% male and 50% female, plus 20% in each of five age brackets, the algorithm would calculate the correct weighting that needed to be applied to each table entry (combining age and gender).