For physical location, Spanish normally uses estar:
But events usually use ser for location:
This often surprises English speakers, because English just says is in all three. In Spanish, a city or office is a physical place, so estar. A meeting is an event, so its location is normally expressed with ser.
A helpful pair to compare is:
Ask yourself: am I locating a thing/place/person, or am I saying where an event happens?
For the location of an event like a meeting, class, concert, or wedding, Spanish normally uses ser. Learners often choose estar because they see a place phrase, but the subject here is not a physical object — it is an event.
Why does Spanish say La reunión es en la oficina but La oficina está en Madrid?
The key difference is the subject: la reunión is an event, while la oficina is a physical place. That is why the first normally takes ser and the second estar. The idea about 'important' location is tempting because learners look for emphasis rules, but that is not what drives this contrast. The building-versus-country explanation notices real nouns from the examples, but invents a pattern that does not exist. The formal-versus-conversational explanation is also a common false trail; this is a meaning difference, not a style choice.