Suspension spring broken

I only broke 1 spring. Why should it always be exchanged in pairs?
If one spring has already broken, the other side will also be worn out because it has had the same loads. If a new, fixed spring and a worn, old spring are driven together, the vehicle will behave out of sync or become crooked due to the different heights. Also: how long will the old spring last if one side breaks? Saving is negligent here.

And consider:
When replacing springs or shock absorbers, always have the track readjusted.