For programming languages that make use of {}, the reason is (almost always) scope.

Take for instance this:

for i in 0..10
do_thing();
do_other_thing();

compared to this:

for i in 0..10 {
    do_thing();
}
do_other_thing();

The intent of the first one is unclear. In the second one it's clear you should loop do_thing() and then run do_other_thing() afterwards. The indentation is only for readability in the above though. Logically there would be no difference in writing

for i in 0..10 { do_thing(); } do_other_thing();

Languages that use indentation and line breaks for scope look more similar to this:

for i in 0..10:
    do_thing()
do_other_thing()