I think the idea is that it strengthens over time and helps to build a sanitized history. When a new generation of people have internalized this national story, then that can then be leveraged by those in power to justify crackdowns and explain away periods of hardship.

In the case of a single party dictatorship, I think it also exists to show party members what the approved line is, which allows local authorities to operate with some degree of autonomy, as long as they can tie it back to the general narrative the leadership wants to promote. This can be used as a way of allowing some degree of variation in governance while still providing the party leadership with plausible deniability if a policy fails (or the ability to claim ownership if it succeeds).