Used for creating items with or without recipes, making nice graffiti and putting mods on weapons. Some of the items you can create require workbenches to assemble. Cooking for example requires a stove of some kind.
Without a recipe, you can try to see if you can make something out of an item using:
And then when you figure out what to make it with:
One way to grind craft is to use the license plate press in prison. Smuggling things in through the glass window in the visitors' lounge is an easy way to get the 4 stars you need to walk into prison. an npc or player in sight of the cameras is another way to get a quick sentence. Even if your stars have disappeared you still need to pay your fine of $0 at the terminal to get your implant removed when it is time to leave prison.
If you don't feel like going to prison, crafting things in your home or HQ grinds craft. Collecting skins from orphans is a good idea as you can use them for various simple craft projects, like making berets. Cooking simple meals, like steak, can also grind craft quickly, so don't throw away the meat from all those orphans you have been slaughtering. Don't queue, tempting as it may be, as this may result in loss of materials. Instead wait for the item to be made before making a new item.