It is generally about concealment and comfort.
IWB will generally allow you to carry a larger gun concealed.
While some may find IWB more comfortable, most will find OWB more comfortable, but not as concealable as an IWB holster.
In a nutshell. In The Waistband covers up more gun from the top of your pants line down. The offset is: You Must Buy Bigger Pants. Usually 2+ up, if you were a 36, you go to a 38, and another offset is, no gun, pants fall down. Empty holster, pants might fall down. Hard holster to keep pants up, more comfort issues. While more concealable, IWB is more difficult to find a comfortable holster and creates wardrobe issues that OWB doesn't deal with.
Any belt wear creates a need for a true "gun belt" to support a pound and a half, or more, of gun and holster. A 1" dress belt will not work, a 1 3/4" belt may not fit the belt loops and can be difficult to lace in. Then you try to adjust it to get a good fit and fight the thick belt to move it an inch around to center the buckle for best fit. (Mil vet OCD)
Belt wear also has limits - someone with a 38" "belt" line may finally realize its at the bikini line when Mr Beer Gut forces the waistline of trousers to their least intrusive position. At that point plumber's butt is the least of your worries, cinching the belt tighter creates pressure on the hip joint where nerves are located and that can be painful just walking thru Walmart. Belt carry is for those who still have abs, not gut. There is a reason some go to small of back which allows some carry but has it's disadvantages.
When the forces of nature and too much nurture combine, and logic wins out, the nonabbers finally resign themselves to carrying with a shoulder holster. That doesn't mean $150 Miami Vice mule harness, there are athletic holsters like Pistol Wear, or on the highly discredited side, black nylon rigs from Chyna which are given shoulder pads etc all which increase them showing. A good holster has semi circular leather cut for the draw side, the holster properly attached, a drop clip to keep in place, and a strong side strap around the other shoulder which is routed in the back, no exposed harness in front.
I have one for a .38 I bought used like that and it's comfortable, absolutely causes no issues with pants or shorts, and can be concealed under a camp shirt - even tucked. It's relatively no harder to draw than a tucked IWB and IMHO certainly a lot more comfortable. I only wear shoulder holsters now. Upsides, it's accessible when sitting at a restaurant or driving, very little interference with a seat belt, works well with coats and jackets especially with reholstering. You can balance the weight with a strong side mag pouch, agents hang handcuffs etc from it too.It's been in use for decades, long before the tacticool precision shooters came up with all sorts of race rigs and started influencing CCW with fast draw McGraw tactics and the "need" for red dots and extended mags - which all interfere with a good draw and also print like crazy.
If you have a waistline 10" smaller than your chest - that's at the maximum girth, not below your hips across the groin - belt will work fine. If not, avoid it like the plague, save your money, because even a 2d tier shoulder holster will be more comfortable than a badly fitting and expensive 1st tier hybrid. As many report here and on other forums - the dreaded box of holsters behind the shoe rack is a handgunners "black rifle disease." It keeps on growing, and at a minimum of $25 a pop up to $125, it can be a significant cash drain.
Now you see why some attempt to pocket carrry. Maybe, its still hanging from your belt in an integral holster sewn into the clothing. Can those pants actually do that? Maybe, maybe not.