There are many ways to use decals in Roblox outside of game development. For example, you can use a decal ID to customize your Bloxburg house: change the floor or place a poster on a wall.
Some games allow to customize your clothing with decals. You can apply them to your T-shirt, hats, and other accessories. Sometimes there are billboards in some games. You can place a place a decal there too.
Visit the Roblox Image ID list to find codes for the most popular images and decals in Roblox.
Please note that if you see an image of a hairstyle or clothing, you cannot wear it. All codes that you see above are decal IDs. They are not actual items that you can wear on your Roblox avatar. Decal IDs are commonly used in Roblox game development or in some games that allow the use decals.
So, if you copy a code for a hairstyle or clothing, you can use it as a poster in your house or on a billboard, but you won't be able to wear it as an item.
Using decals in game development is a different story. If you're a developer, I don't need to explain this to you. You definitely know how to use them.