What problem does this solve or what need does it fill?

In order to add a unit struct to a components required components you need to implement Default on it, which is unneeded as unit structs only have one state.

What solution would you like?

Ignore the Default requirements for unit structs

#[derive(Component)]
pub struct Player;

#[derive(Component)]
#[require(Player)]
pub struct Character {
    health: u32,
}

What alternative(s) have you considered?

Deriving Default on all unit components which is a bit tedious.