This example illustrates the previous paragraph
median(x = 1:10)
#> [1] 5.5
x
#> Error in eval(expr, envir, enclos): objeto 'x' no encontrado
median(x <- 1:10)
#> [1] 5.5
x
#> [1] 1 2 3 4 5 6 7 8 9 10
Created on 2019-03-27 by the reprex package (v0.2.1)
When working at the top level (like in your example) there is no difference, although is customary on the R community to use <-