I hope you found this post because you asked the same questions and don't have an answer, yet.
Yes, I do use DNA. I spend hours and hours using DNA.
It mostly gives me more questions and no answers.
But it's still really cool (otherwise, I wouldn't spend hours on it).
I'm going to give you the same kind of answer I gave the other night. It wasn't technical or long. I was standing at a barre at the time (yes, that's spelled correctly). There wasn't time for in-depth explanation and that's probably not what you want, either.
If you take a DNA test for genealogy, you will need a family tree. You will also need other people that took a DNA test and their family trees. If they don't have family trees, you will have to do the research and create them (and that's the highlight of most of my "DNA work," I'm such a genealogy geek).
But let's start with something you are probably familiar with, a paternity test. This is probably the root of most people's misunderstandings about using DNA for genealogy. This isn't CSI. You won't get a piece of paper with all your answers. But the process is actually exactly the same, you missed some important "factors" when you were watching CSI (or Jerry Springer, wherever you learned about paternity tests).
With a paternity test, two (or more) people are tested. It is a different kind of test but we're not going to worry about that---simple and quick, remember? A paternity test, or other DNA tests for relationships, ala CSI, start with a hypothesis. One testee is believed to be the father (or other relative) of the other testee.
In this case, you test two or more people to see if they are a certain relationship. In genetic genealogy we usually say "they share a common ancestor." This is like a paternity test (one testee is the ancestor of the other, or perhaps two potential siblings are tested, one or both parents could be the common ancestor).
Here's where genetic genealogy gets different from what you imagine from your Jerry Springer binge. Genealogy DNA tests can't "tell" the exact relationship if the shared ancestor is too long ago. "Too long ago" is reached very quickly, in fact. A paternity test can't either. It's called a paternity test for a reason, that's what it can guarantee.
In addition to the "reach" of the test I'm guessing you didn't consider the following "factors" when you think about DNA used on t.v. versus DNA for genealogy.
A paternity test starts with a lot of information about the testees. You don't just grab two people off the street. As I said, you have a hypothesis. That hypothesis is based on facts. For example, you don't test to see if a 44-year-old man (A) is the son of a 25-year-old man (B). That's not possible. For paternity, the parents also had to come in contact in some way. So if you're testing if someone is the father, he has to be male, he has to be old enough to be the father, and he has to have been available, location-wise, to be the father.
That's the start of a family tree.
Genetic genealogy requires the same type of details. If you are testing a hypothesis, you need to know about everyone involved. In genealogy, that includes non-testees in the family tree.
You research the deceased ancestors and exclude options that are impossible based on age, sex, residence, conditions, etc. For example, dead men (historically) don't father children. There's no point testing a hypothesis that a little research would have excluded because a man's "condition" was "deceased" long before the child was born.
In genealogy, there's much more chance a complicated relationship will skew results than in a modern paternity test. The way to be aware of a complicated relationship, or determine there isn't one, is through traditional genealogy research.
If you are a CSI fan, remember, there's another kind of DNA test you'd often see. They'd run a sample through CODIS. Once again, different kind of test, and different purpose. But similar concept to genetic genealogy. This is what most people do for genealogy.
When you take a genealogical DNA test, what you get back is essentially a list of matches (you get additional information but that's not directly related to this discussion). On CSI they were usually hoping to match a DNA sample to DNA from the same person. For genealogy, you want to find people with a shared ancestor but you lack the hypothesis to know who to test. Guess what, you need information from a family tree to make it all work.
If you take a genealogical DNA test, you won't get your family tree as your results. You will get back a list of matches. You then need to use the matches, the DNA data, and everyone's family tree to do analysis. Exactly what kind of analysis differs with the type of test (currently there are four types) but they all involve having a well-researched family tree for you and for your matches.
I think I've answered the question "do I need a family tree if I take a DNA test?" If you were thinking DNA would be a shortcut so you didn't have to do any "research," I'm afraid you're wrong. However, if you're like most genealogists, research is fun. Now you can research your matches' trees as well as your own. The answers won't be quick and easy but DNA is a powerful addition to traditional genealogy research.