Let's take an example of adding a value to a range (as it could be done in a loop for example):
3+1:5
Gives:
[1] 4 5 6 7 8
This is because the range operator : has higher precedence than addition operator +.
What happens during evaluation is as follows:
3+1:5
3+c(1, 2, 3, 4, 5) expansio...