This is an idea that probably shouldn't be implemented because it adds unnecessary complexity and has a perfectly fine workaround. I'm adding it anyway as I may be the only one that believes it may be unnecessary.
Like many, I use the Data Viewer to work on complex expressions before implementing them. I have so many, I cannot remember what their purpose is. It would be handy to have a name or comments area to put extra information in. The perfectly fine workaround is to put a comment as the first line in the expression.
I am not voting this idea up myself at this time. If user defined names are added to the table names, it might make some sense to also add it to these expressions.