Well, a little history....
Tags are a historical artifact, like your appendix. Unfortunately, they are gaining in popularity.
Most SCADA systems trace their roots to DCS in the process industries, where every field device had a tag name and tag number. These tag numbers were derived from adn shown on the Process and Instrumentation Drawings. When SCADA came in, because the process and DCS engineers were used to thinking in terms of tags, they included tag names in the set-up portion of the I/O being read and displayed. The scaling, addressing, register location, data type, etc. were all part of the tag data base.
When PLC O/I devices started showing up they were generally register based, having evolved from timer access modules and hand held programmers and such. You just plug in the data register and PLC address and you are done.
SCADA software for some reason has influenced O/I software rather than the other way around. I suppose the more complex O/I software stole programmers from the SCADA folks. At any rate, more and more O/I seems to be tag based. I personally think it takes longer to set up and maintain a tag based configuration than one with straight register addresses, but there may some hidden advantage to tags.