How does this work?

Saw this in a macro by the GSE author, any1 know how it works?

/cast !Prismatic Barrier

Notice the “!” in front of Prismatic

So how is different from:
/cast Prismatic Barrier

I do know that something like:

8 !== 9 means 8 does not equal 9 so how is the “!” used in GSE (or what does it represent, as I still get my Prismatic Barrier)

It now serves no purpose.

Good to know but can you enlighten me a bit more, what purpose did it server before?