What I think Chapter 5's narrative is doing. [Spoilers]

A lot of people have complained that Chapter 5 didn't really progress the main plot forward. To an extent, this is true, but I think people are missing the trees for the forest.

Chapter 5 is the most overt reference to a number of things previously either hidden or implied. We've had years to comb through every detail, but a casual blind playthrough of a complete Deltarune wouldn't.

If someone hadn't been gathering the Shadow Crystals, this chapter is the first reference to the Black Shard. If someone hadn't fully explored the Holiday home in CH4, this is the first reference to what happened to Asgore in the past.

It may not be bringing much new to the table, but it's bringing previously hidden things to the forefront. Both of those examples are on the main path. There are a handful of other examples, too, that people may have taken for granted, like confirmation of Ralsei being capable of using fire magic.

This is, of course, in addition to the main narrative of Chapter 5, which is more character-driven, focusing on the growth of the cast (especially Ralsei, but Susie to an extent, too).

It's spelling out the fact there is a mystery before we get into the last act, meaning that first time players who haven't spent a decade taking a fine tooth comb to the game's files can appreciate what's going on too.