if you have

s: STRING

s := "test"
s := s + "123"
s := s + "456"

you should prefer `append'
otherwise, you will create many intermediate object for nothing.
in a similar way
it is better to do

s.append ("a")
s.append ("b")
s.append ("c")

rather than
s.append ("a" + "b" + "c")
since "b" + "c" create an intermediate object, let's call it bc
then  "a" + bc  create another object let's call it abc

BUT here, this is tiny detail, but even if in many kind of app, this does not really matter, for web server it is a good practise to try using as little memory as possible.

but once more here this is not critical, when concatenating big text .. this may become more important.