Here's why:
Concrete:
* When referring to the physical building: "The protesters marched towards the Congress building." Here, "Congress" refers to the specific physical structure.
* When referring to the gathering of people: "The Congress is in session today." Here, "Congress" refers to the group of people assembled for the purpose of legislating.
Abstract:
* When referring to the concept of legislative power: "The separation of powers ensures Congress has its own role." Here, "Congress" refers to the broader idea of legislative authority, not the physical building or the people within it.
* When referring to the institution as a whole: "The Congress is responsible for passing laws." Here, "Congress" is used as a general term for the legislative branch, encompassing its structure, function, and authority.
So, the answer depends on the context and how you are using the word.