When creating slab text block, for example, as a single text frame (rows with different lengths of text, and make the rows equal lenght by changing the font size of each row, rather than changing the tracking) some of the rows you want to overlap other rows (big font sizes) as a design feature, for example titles as slab text block.
In that text block you could push selected text in the background, behind the not selected text, if this feature would exist.
So that you wouldn't be required to create a separate object for each row of text.