Why is the shovel operator (<<) preferred over plus-equals (+=) when building a string in Ruby?

Question

I am working through Ruby Koans.

The test_the_shovel_operator_modifies_the_original_string Koan in about_strings.rb includes the follo

Answer 1

Proof:

a = 'foo'
a.object_id #=> 2154889340
a << 'bar'
a.object_id #=> 2154889340
a += 'quux'
a.object_id #=> 2154742560

So << alters the original string rather than creating a new one. The reason for this is that in ruby a += b is syntactic shorthand for a = a + b (the same goes for the other = operators) which is an assignment. On the other hand << is an alias of concat() which alters the receiver in-place.