let's go back already
давай наконец вернемся!
ALREADY in this usage is a particle used with imperatives that expresses impatience. In this case, for example, the speaker may have suggested going back several times and nobody paid attention. In another example, if a woman repeatedly asks her husband to take the dog out and he doesn't do it, she can say "Take the dog out already!"
let's go back
Editadoобычно говорят без (already). Пошли назад/Давай возвращаться.
обычно говорят без (already). Пошли назад/Давай возвращаться.
Elena, to remove ALREADY from this example is to take away the emotive quality of this type of imperative. It has a definite place in English expression and isn't "usually" omitted - people use it when they mean it and the learner should know what it implies. So excuse me if I don't agree with your assessment.