You should test more Y-chromosome DNA short tandem repeat (STR) markers when you wish to compare additional markers against others with similar results. They will refine your matches for genealogical purposes. It does not increase the number of generations traced by a Y-DNA STR test but rather reduces the range of generations in the time to a common ancestor with your match.
Testing additional STR markers can also help refine the matches in the Y-DNA – Ancestral Origins page. Testing more markers means that the information is more relevant to your personal ancestry. However, information on deeper origins (historical and anthropological) may be better answered with an extended Y-DNA SNP test like the Geno 2.0 test.
If you do not have many matches or do not have many matches that have tested more markers, you may still choose to upgrade. That way as the database grows and you gain additional matches, you will be prepared to make comparisons that are more refined. Note that our Y-DNA111 is new enough that matching and information from matching may be limited by our current database size. This test is becoming more popular and the database is growing rapidly, so this should change over time.