I dont understand the need to show the past of that pastry chef and him.

Idk for the author it may have been just that they liked the character and wanted to do it

For me, as a reader, I understand the story to be much more about maturing and changing than it is about their romance. Hence showing Mako's first time making such a resolution for himself, while also portraying the people around it cuz everyone is changing, not just the main ones