Since a lot of trans people find they have significant dysphoria about their lower regions, having something down there can help ease the stress for some. There are a lot of options out there depending on what a person wants from a packer and/or can afford. You can go from anything that simply adds something there (like a rolled up sock) to expensive, hyper-realistic prosthetics that can stay on you with adhesives, may also allow you to "stand to pee" (STP), and/or have a "play" feature. It's pretty amazing.

Besides using adhesives (which isn't as common), the two other ways people wear a packer are either tight-fitting underwear (like smaller boxer briefs) or a "harness". The harness is essencially a jockstrap, sometimes with a hole/opening for the shaft, particularily for packers with an STP or play capability. Really, it's just anything that works to keep stuff in place.