What will central government do once decentralization has taken place?
Currently, most services (education, healthcare, rural water supply etc.) are provided by line ministries (central government). They also do most of the planning and construction of service outlets, like schools, clinics and water points. This will change with decentralisation. In future, line ministries will be more focussed on planning, training of personnel, setting standards, and monitoring ("making sure that things are being done properly".)
In short, decentralisation will leave central government free to deal with matters that affect the nation as a whole – the defence, for example, will always be controlled by central government because it has to defend the whole country, and not just one region or town.