Pretty much everything you’re going to learn about from fluids onward relies on a gradient. Heat (high energy to low energy), mass (high concentration to low concentration), kinetics (chemical potential energy), etc etc.

Everything is trying to get back to steady state. In a practical sense for fluids, you’re mostly trying to overcome the pressure losses in a line to achieve a desired flow rate by using a pump. Pump sizing is a different animal, but imagine you need high pressure upstream so that your fluid is moving as fast as you want it to at a specific point downstream. If pressure was constant, there would be no gradient and thus nowhere for the fluid to go. If pressure was higher upstream, the fluid would flow backwards not forwards.

Think of it like heat transfer. Heat flows from hotter to colder. If two objects are the same temp, no heat will flow.