Used to be describes a past state that is no longer true now.
Think of a shop sign: This used to be a bakery. It was one once, but it isn't today.
The office used to be much smaller. It was smaller in the past, and it is not smaller now.
English used to be optional for this role, but now it is essential. The sentence lines up the old state with today, which is what used to be does.
Not the same phrase as be used to
Quick boundary note: be used to looks similar but works differently. It is followed by a noun or an -ing form and means something feels familiar or normal now, as in She is used to the noise, which says the noise feels normal at present. Used to be, by contrast, describes a past state that has changed. Keep the two apart rather than reading them as one pattern.