STR testing measures relationship in a time frame of hundreds of years, making the distance to the MRCA (Most Recent Common Ancestor) sufficiently close to be genealogically useful.  And "useful" is an understatement.  STR testing is a powerful tool for determing whether people do or do not share a recent common ancestor.

There is another form of Y-chromosome DNA testing called SNP testing — the acronym stands for "Single Nucleotide Polymorphism" and is pronounced as a single word, "snip."  Results of SNP testing are expressed as + or - (positive or negative) to indicate the presence or absence of a particular mutation.  Each suspected mutation requires a separate test.  Results of SNP testing determine a person's haplogroup, and deep testing (meaning exhaustively testing all known SNPs for the haplogroup) can identify subgroups, called "subclades."

The haplogroup (sometimes abbreviated, "Hg") is a measure of deep ancestry.  The MRCA may be thousands of years in the past, which places the individual's ancestry in a paleoanthropological time frame.  Haplogroup determination is of practical use for the project because it broadly defines and separates the most distinct family groups; that is, it separates families with no prospect whatsoever of a connection in "genealogical time," making it a useful way to divide up the project.  For the individual, knowing one's haplogroup subclade greatly enhances one's appreciation of history because the literature on the genetic history of humanity, correlating haplogroup subclades with historic and pre-historic events, is growing rapidly.

Speaking personally, compiling my genealogy greatly enhanced my appreciation of U.S. and European history, while discovering my parents' haplogroups has enhanced my appreciation of the global history of humanity.  I wish I had known these things about myself when I was growing up, but at least the future children in the family will know.

As each individual has just one evolutionary path on their patrilineal line, their SNP-based haplogroup should correlate with their STR-based haplotype, and it does.  If the haplotype is reasonably common (or similar to one that is), it can be used to deduce the probable Y-DNA haplogroup, without the added expense of SNP testnig.  If the haplotype does not unequivocally indicate the haplogroup, FamilyTreeDNA will do a "backbone" (basic) SNP test without charge to securely determine the basic haplogroup.  Deep SNP testing would still be needed to determine the haplogroup subclade, and if your haplotype is rare, I recommend deep SNP testing simply as an aid to researchers because their research on your haplogroup ultimately tells you more about yourself.  Haplogroup determination is (to me) a fun thing to know about oneself, but it is not a requisite for full participation in this project, which is based on the results of STR testing, not SNP testing.