Choose investment assets that match future money goals else encashing them at time of need can be a problem

We would not care about financial planning, or assets, or asset allocation, if all we needed to do was receive income and use it to spend as needed all through our lives. Imagine money to be a set of pipes, fed by a regular source of water, available to open and use as needed. Why would one think that a water tank to store water may even be needed?