In comments under the question about the UK's refusal to apologise for the Amritsar massacre there has been some discussion in the comments about whether there is any point in apologising for something that has happened very long ago, so much so that many of those involved are no longer alive.
Another point that hasn't been raised in the comments but that I always found a bit weird is that it's not the actual perpetrators that apologise, instead it's a new generation of politicians that feel obliged to do so.
The problem is not just limited to the massacre in the question, indeed this question comes up in different countries and with different issues.
For example, the question about whether the Dutch should apologise for their part in the slave trade (the most they have done is "recognise the injustice" and "express deep regret and remorse").
Another example is the discussion about 'reparations for or the descendants of enslaved men and women' in relation to the slave trade in the US.
Another example is the US' apology for the overthrowing of the Hawaiian monarchy in 1893, 100 years after the fact.
What arguments do those in favour of apologising for massacres and injustices that happened very long ago use? Why is it worth apologising, even so long after the fact?
Note: that this question is asking about arguments in general, not related to any specific injustice or a specific instance of wrongdoing. Of course, examples relying on specific cases are still very much welcomed.