Why log(RR) instead of RR?

As an example of how the variances of difference measures are calculated, the derivation of approximate variance or SE for RR is shown as follows (uses multivariate delta method):

Comparing Risks

To evaluate whether an exposure is at all contributing to risk of disease, the simplest form of comparison is as follows:
x1 ~ Binomial(n1, pi1)
x2 ~ Binomial(n2, pi2)