When dealing with frozen strings, << breaks. Using += unfreezes the string and is thus safer.

[1] pry(main)> msg = 'x'.freeze
=> "x" 
[2] pry(main)> msg.frozen?
=> true
[3] pry(main)> msg << 'y'
FrozenError: can't modify frozen String: "x" (FrozenError)
from (pry):3:in `__pry__'
[4] pry(main)> msg += 'y'
=> "xy" 
[5] pry(main)> msg.frozen?
=> false