Technical documentation is required for all team members to have same understanding about the system and also to see how effective and efficient the model, to be easier for you to see how the system can be integrated to other system or for enhancement, to be easier for you to check wrong process or data flow.
Technical documentation at least should explain:
- Data flow
- Process flow
- Business Logic
Data Flow
Data flow is to explain how data flow to/in system start from input, process inside the system, and how data flow from the system as output.
The concept of all system always same: Input =>Process=>Output
From data flow I know how data flow start from data source to the system and how the relationship between cubes/tables (for RDBMS)/dimension/database.
Process Flow
Process flow is to explain what processes inside the system, and what each process does. From here we will know how is the data processing. When you need to do enhancement, or to check what process impact to the slow performance or to fix/modify something, you can refer to this documentation.
Business Logic
Business Logic is to list the logic inside the system Usually I insert the business logic in data flow and process flow, so I will know how the data and process flow and what logic inside the system once I see the diagram.
Veronika Rotua Gultom 
Once again great post ,It would have been great had you been delve deeper in to the intricacies of technical design.TD is a blue print of the system one implements and It gives end users as well team members fair bit of an idea result set they gonna get.The document should also encompass what affect or impact in terms of value additions to application owners goals or target.I really think TD has more depth and function to it.
Thank you Abdul, unfortunately during my work on some business performance management firms, there was no technical documentation, which was make me difficult to understand the whole system (especially for big system) when I joined in the middle of project or even after system completely done, but need to do support or enhancement. I had to go inside the code (TI & rule) to understand the flow (data & process). Now, I always create this kind of TD, and usually the PM will be happy to see my TD 😀