In general is the albumartist (one word if you refer to the tag im mp3tag) the superior grouping criterion over artist.
For many albums both fields will contain the same data.
In compilations though, the albumartist very often becomes "various artists" and the artist then shows the actual performer.
if the albumartist is not filled then some players pretend that all tracks come from the same artist (the "empty" one) and show each track as track of a separate album.
So, always fill the albumartist tag even if it means to duplicate the data.
In the FAQs you will find instructions on how to create an action that copies the contents of one field to another.