Why do physical resources need to be maintained?

If you mean Personal, then a good diet and exercise is required. If you refer to business infrastructure, the answer is obvious. Plant and equipment, walls and floors, parking lots and wiring; all need regular and careful inspection and maintenance, replacement when necessary. The tax expense known as Depreciation is an attempt to accrue funds within the business to write down the cost of an Asset over it's expected life, or Tax Life as some policies allow, and through the reduction in taxes this allows, create funds for the replacement of the asset. Different assets have different life spans.