Manufacturing is important because:
1) It requires the nationwide creation of efficient and effective infrastructure, in order to be competitive.
2) It provides mass low-skill employment, which is especially important in countries with large populations.
3) It leads to urbanization, which is an essential stage in development, because it takes people out of a subsistence rural lifestyle and brings them into modernity.
Those are the main points, but also:
4) Manufacturing is a way for low-skilled workers to add large amounts of value to raw materials. In addition, the finished products can also be exported to earn a significant cash flow in the form of trade surpluses, freeing up national finances from budget constraints. Which allows additional investment in infrastructure/education/etc.
These are the reasons why almost all of the current developed countries once went through their own large-scale industrial stage.