Why are projects divided to subprojects?

"Divide and rule". A project divided to subprojects is a kind of Program. Program management is different than project Management. Here you have one PM while there you have more than on PMs. A project is divided to Phases, Tasks, Sections etc etc depending on nature of work and field.

For a better and effective management

In order to run a successful and effective management

For successful results.

In project management terminology, a project is divided into phases and not projects. on the other hand, a group of related projects constitute a program.

now, projects are divided into phases for many reasons. in practice, here is why project managers like to divide a project into phases:

- Its important to know each phase importance and contribution to overall project success and therefore resources and cost centers are prioritized on that basis.

-slicing a project into smaller pieces helps a project manager focus more on competing the current phases keeping in mind its contribution to overall project progress.

-dividing a project into phases helps in the planning process when scheduling and costing is performed. relationships between phases are defined and durations are calculated easier.

-dividing a project into phases can be of great help when fast tracking or project crashing is performed. a project manager is able to run different scenarios to try and understand impact of increasing resources on project completion.

slicing projects into phases is a standard practice today and project managers decide where to draw the line based on distinct attributes between phases that can mark the end of one phase and the beginning of another.

hope I could provide some insight towsrds answering you question.

For optimum planning, execution, monitoring and controlling by dividing to packages then activities for ease of trace with presenting the key performance indicators per sector with aggregating its accumulation vs. the baseline.

For the same reason that managers usually have supervisors leading the line workers.  A manager's focus is more strategic in nature and a supervisors role is more focused on one aspect of the strategy.  The wise manager will actually attract supervisors who are better suited than the manager himself to lead a specific team of line workers in a specific task.  Similarly, the wise project manager will attract focused leaders to handle the sub-projects in accordance with his overall direction and strategy.  For example in a complex construction project, the project manager will have an electrical supervisor that probably knows far more about electrical construction issues than the manager himself.  The project manager will rely heavily on the expertise of the electrical supervisor to handle the electrical sub-project.  

For easy handling and successful completion,

Because the Sub-Projects are independent in themselves , however their outcome or direction decides the direction of the main project.

Also when the main project is too large or complex , it can/may be divided into Sub-Projects.

Mainly: To comply with the time schedule and exercise cost control.

You can read on work breakdown structures also. What it means is breaking the work scope into smallest units which cant be broken down further and assign owners to each unit. that way you know the progress on individual unit and the complete work. Also a tip would be to assign the % that completion of each unit will contribute to whole project.

Ex: Construction of Town Ship:

Bricks and cement (20%) - Identify Stakeholders and Owners (Suppliers)

Block A (10 levels building - Selling of flats will generate revenue for other buildings)35% & Priority

Manage Resources (25%) Ensure the vendor is supplying resources always ( At given point of them you can have hit of3% only) maintain daily report from supervisor and fix ASAP.)

Start slow work with other projects (20%)In above ex. If the priorities and sub projects are not defined. Resources will spend time equally and the cost might increase when the project is complete.