One of the purpose of a boiler is to induce phase change, in other words boil the fluid. Now If you’ve ever tried cooking in areas of higher altitudes you might have noticed that boiling point would have decreased and it took a lot longer for you to cook (since Turning up the heat will not make food cook faster because liquid cannot exceed its own boiling point.).
Similarly, it’s absolutely vital for proper functioning of a boiler to run at a predetermined pressure so that the behavior can be predicted and components in the boiler are designed accordingly.
Take a look at process 2–3 in a Rankine cycle
Water never changes from a specific pressure.
Temperature on the other hand isn’t really maintained and does change(except during phase change).