The importance of technical documentation on an abstract example:
Imagine that you are engaged in the installation of telesystems: you connect equipment for corporate clients. You agree with the client that you will come to his office to connect the equipment. In the usual sense, this is to arrive, install computers and stretch cables from the outlet to the machines for connection. The client may think that you will come, run the cables from the box to the office on the floor, then mount the sockets for connection, and only then connect the equipment in the office. Agree, your understanding and understanding of the client, in this case, differ significantly.
The essence of technical documentation is that everyone understands each other. If you used a list of services fixed on paper and showed it to the client before signing the contract for the provision of services, you would quickly understand each other.