Why do we say "D'ouest en est" but also "Ceci est situé à l'ouest, et cela est situé à l'est". Can someone please explain when to use "à l'est" and "en est"?
To start, please read this to understand difference between "ouest" and "Ouest".
You can see there is an ambiguity between the East as a region and the east as a direction. In French you would say "en Est" in the first case and "à l'est" in the second case.
Actually "en Est" is less and less used in French, to the point you don't really hear it outside of expressions like "d'Ouest en Est" or in tabletop games terminology. People uses "dans l'Est" instead when referring to the eastern region.
"D'Ouest en Est" suggests a movement from the West to the East, something like a migration or a weather phenomenon. The phenomenon is observed with a global point of view. Strangely enough "De Nord en Sud" is not used often at all ("Du Nord vers le Sud" is used instead)
"De l'ouest à l'est" suggests a change in the orientation, like the movement of a radar trying to get a signal from a wide angle. The phenomenon is observed from a local point of view.
To state the location of something, you can say:
Ceci est situé à l'ouest.
It means from where you are, walking toward the west, you will eventually get to it. But you can say, too:
Ceci est situé dans l'Ouest:
It means it is in a western region. Not particularly from where you are, just considered as objectively western. Of course it is silly because nothing in the Earth is more western than something else, but due to francocentrism that can be used to speak about the America, or the "Far West".
You can also say about someone "il est à l'ouest", which means he is disconnected from reality, but that is not very relevant here.