Technique should be secondary to the final product. What is your conception? What do you, as knitter, want to produce? If I am working on a school sweater for a young, athletic child -- lots of running, playing, and moving involved -- I use durable yarn, lots of acrylic, machine washable and machine dryable, and a design that allows for movement. Usually a knit-in-the-round design, maybe using cut armholes or cut front for a cardigan. For a young woman's formal wear, I might use a tailored design, done flat on two needles and with seams to retain the shape of the sweater. The yarn is usually a fine yarn in luxurious fiber, perhaps cashmere, alpaca, or mohair. It all depends on the use intended for the end product.
Along this line of logic, needles come in various styles and forms related to accomplishing a particular task. The two biggest groups are straight needles and circulars. Which will be appropriate depends on the concept of the garment.
Straight needles are used for sweaters that are knitted as flat pieces. Knitting back and forth requires knitting one way and purling back. Many people don't like to purl because purl rows don't always have the same tension as knit rows and the sweater may look uneven. By dropping flat work from their repertoire, these knitters lose the benefits of fitted sweaters and set-in sleeves. In addition, the whole field of intarsia (blocks of color) becomes much harder to achieve.
Sweaters knitted as tubes require either double pointed needles or circular needles. Sweaters done in the round are flexible and move with the user. Sometimes, the shaping goes wrong and some yarns will sag and lengthen over time. Without any seams to hold the shaping, the sweater gets longer and thins over the shoulders. Most in the round designs flatter wearers with rounded shoulders and don't go well with wide, squared shoulders. Body type should be considered when thinking about this design.
The versatile knitter probably will work on sweaters both flat and in the round depending on the garment and design. And so will need a full set of needles, circular, double pointed, and straight. (Eventually.)