There are a number of reasons why you should obtain a permit, but first and foremost, for the safety of you and yours within your property.
If your project requires the submittal of plans, a Building & Safety plans examiner will review the plans to verify the project will comply with local building and zoning codes. Once approved, a permit can be issued and construction can commence. From beginning to end, a building inspector will inspect and verify that the work is being done in conformance with the approved plans and building codes.
In addition to the above, there are legal and financial liabilities that you could face if you weren't to get a permit. Work without a permit is illegal and could pose serious complications for you when trying to sell your house. Any fire and homeowner's insurance you have may be invalidated if you do work without a permit.