What does the %<>% operator mean in R?

Question

1. What does the %<>% operator do in R ?
2. What's the difference between using %<>% and <- ?
3. In what type of circumstances %<>% could be useful ?

Answer 1

The help, ?magrittr::`%<>%`, answers all your questions, if you are refering to magrittr`s compound assignment pipe-operator:

[...] %<>% is used to update a value by first piping it into one or more rhs expressions, and then assigning the result. For example, some_object %<>% foo %>% bar is equivalent to some_object <- some_object %>% foo %>% bar. It must be the first pipe-operator in a chain, but otherwise it works like %>%.

So

library(magrittr)
set.seed(1);x <- rnorm(5)
x %<>% abs %>% sort
x
# [1] 0.1836433 0.3295078 0.6264538 0.8356286 1.5952808

is the same as

set.seed(1);x <- rnorm(5)
x <- sort(abs(x))
x
# [1] 0.1836433 0.3295078 0.6264538 0.8356286 1.5952808