Let me start by saying this is a loaded question. We will probably have a lot of different opinions here and I would love to hear them so please drop a note in the comments with all your serger feelings.
A serger is sometimes called an overlocker, depending on where you are in the world. They are simply different names for the same machine.
Back to the question, Do I need a Serger? The simple answer is no you do not. You can basically sew anything you want without having a serger, knits included. So the better question is - Should I get a Serger? And my answer to that is Heck Ya! Order it today if you can. It is a bit of an investment, on average they can run about $300-$600. So if you have a sewing budget start saving for it because this little machine will change your sewing life. The one I use is the Juki MO654DE and I love it.
I realize that this machine can seem intimidating and scary at first because let's face it, the machine has 4 threads and a knife built in. I actually waited several days before unboxing my machine when I first got it because I didn't want to learn how to use it.
First, the knife
It's there to cut off any extra fabric so you get a nice clean finish. Don't be scared of it, but stay on the safe side and keep a little distance between your fingers and the knife when serging.
Second, threading the machine
Ok I'm not going to lie, it is a little crazy the very first time you thread your machine. Mostly because you have to get into tiny areas and use a tweezer to get into some of the trickier to reach places. But, it's really easy once you get the hang of it. I would suggest threading and rethreading it at least 10 times when you first get your machine and you'll be set!
You're ready to start serging!
When serging knit fabrics there is no need to use a straight stitch machine at all. Just go straight to your serger and start sewing as normal. The stitch a serger makes looks like a whole bunch of tight zig zags so it will easily stretch with your fabric. The stitches will not break when you stretch or pull on your fabric. Making this the perfect machine for sewing with knits.
What to watch for...
Be aware that there can be wavy seams when serging knits or stretch fabrics.
When buying a serger try to find one with the "Differential Feed" feature. It basically moves the top fabric and the bottom fabric through the machine at different rates. This helps control the amount of waviness in your seams. The normal setting is "N" you can move up from there or down from there. Here I'm serging on a pretty heavy weight sweatshirt fleece and when set at "N" you can see the seam is wavy and the stitches are tight together.
For wavy seams like this start moving your dial up until your seams lay flat. See the difference when I move up to 1.5. Use the higher settings for thick knits and extra stretchy fabrics. Don't worry if it's not perfectly flat you can always steam out any extra waviness with your steam iron. In case you are wondering the lower numbers are generally used for fine nylon jersey, closely woven fabrics or thin linings and satins.
I don't think there is much more to know about this machine. Just sew like you would a with normal straight stitch machine, a serger is just making a different type of stitch. So start serging and experiment with a bunch of different materials. Like all machines, when introducing different fabrics you'll need to spend a few minutes figuring out what combination of settings you like best.
Overall, I think you'll end up really loving the serger. It's fun to use and sews really quickly making projects like T-shirts, sweatshirts and joggers a breeze. We took our Collage Gather Top that is originally designed for woven fabrics and decided to serge one using left over french terry. We gathered the sleeve bottom by double folding the hem creating a channel for inserting elastic. If you are looking for a cozy knit top give this a try.
We hope you dive into the amazing world of knits with the help of a serger. Can't wait to see what you make!