The more people on your family tree that have their DNA tested, the more you are able to round out your family's haplogroup DNA "family tree."
For example:
1. Me:
Mitocondrial: U2e1a
2nd Generation
2. Dad
Y-DNA:
R1b1b2a1 (23andme) or R-M269/R1b1a2* (FamilyTreeDNA)
Mitocondrial: H
3. Mom
Mitocondrial: U2e1a
3rd Generation
4. Dad's Dad [Charles]
Y-DNA: (see #2)
Mitocondrial: Unknown
5. Dad's Mom [Eloise]
Mitocondrial: H
6. Mom's Dad [William]
Y-DNA: currently unknown
Mitocondrial: Unknown
7. Mom's Mom
Mitocondrial: U2e1a
4th Generation:
8. Dad's Dad's Father [Frank Seraf]
Y-DNA: see #2
Mitocondrial: Unknown (his sister had only sons; his mother's sister had only one son)
9. Dad's Dad's Mother [Hazel]
Mitocondrial: Unknown (no daughters, only one son living)
10. Dad's Mom's Dad [Byron]
Y-DNA: Unknown (no living male descendants; his brother did have male descendants that I think are living)
Mitocondrial: Unknown (no sisters; his mother had no sisters with descendants)
11. Dad's Mom's Mom [Mina]
Mitocondrial: H
12. Mom's Dad's Dad [Warren L]
Y-DNA: currently unknown [Warren L did have male siblings whose sons could be tested]
Mitocondrial: Unknown
13. Mom's Dad's Mom [Mary Jane]
Mitocondrial: Unknown (no living direct female relatives to test)
14. Mom's Mom's Dad [Frank]
Y-DNA:
R1b1b2a1a2f* (23andme) or R1b1a2a1a1b4/R-P312 (through conversion for FamilyTree DNA equivalent)
Mitocondrial: Unknown
15. Mom's Mom's Mom [Mary]
Mitocondrial: U2e1a
I wanted to show that I got the haplogroup of #14 through testing of second cousin via 23andme, even though I had no one in my immediate family that could have done that test.