Why would u ever need to devolve

Why would ash want his pikachu not to evolve

I think I know why

why then?

If the evolved form requires huge amounts of energy, it may be difficult to sustain. Being able to revert allows you to keep a more powerful form, despite the huge amounts of energy it consumes, while not having to deal with the effects by simply changing back. Think of it like the SSJ from DBZ.

Or it may be for compaction. Imagine if your garchomp could de-evolve into gibble and then someone challenges you. Then you could just have it poof into garchomp and it’s a win.

Indeed