Quite simply, because it is a cost-effective way of mitigating the substantial risks associated with this highly important artifact.
Architectures are the blueprints for a system, and the carriers of the system's quality attributes.
Most complex software systems are required to be modifiable and have good performance. They may also need to be secure, interoperable, portable, and reliable. Experience has shown that the quality attributes of large software systems live principally in the system's software architecture. In such systems the achievement of qualities attributes depends more on the overall software architecture than on code-level practices such as language choice, detailed design, algorithms, data structures, testing, and so forth. It is therefore a critical risk mitigation measure to try to determine, before a system is built, whether it will satisfy its desired qualities.