This is a cool little trick that I use a lot because of preference. The idea is that you can rename all the fields within your Qlik application using one statement, two if you count prep work.
I tend to use this so that the fields in the data model are the same as the front end. This was something I started doing because previously in Qlik Sense, even if you named a Master Item differently, the field name showed up in the breadcrumb trail. Now it is not so important.
I also do it at the end of the load script so the field names can stay consistent up to that point. This makes troubleshooting a lot easier on both sides.