Proportions (and other rates such as percentages) are often useful in statistics for comparing samples with unequal sample sizes. For example, suppose we have two samples, one of men and one of women. The sample of men contains 1024 men, and of them, 558 claim to be Republicans. The sample of women contains 886 women, and of them 422 claim to be Republicans. It would be silly to compare the numbers 558 and 422 because they come from different size samples.
The solution is to calculate proportions (or percentages). Among the men, the proportion of Republicans is 0.545 or 54.5%. Among the women, the proportion of Republicans is 0.476 or 47.6%. Thus, relative to sample size, more men are Republicans than women.