I use PC as my platform, so to use a sign you place it, then equip whatever item you want displayed on the sign, then use the item on the sign (mine is right click). The item won't be consumed in the process, either, so you can put whatever your want. I use signs for my fruit trees to label them when they're off season, just so I know which is which. It also helps if you have a big farm, because early in the season the seeds start to all look the same/similar so it helps to use signs to label them until they grow more, and it makes the farm look pretty and organized. Signs are great for organizing chests of you have a lot of them, so you could put signs next to a chest/row of chests to indicate what's in them without having to memorize them or give them color schemes. Basically, just use the signs for labeling things you need labeled so you don't forget what they're for. Hope this helps! (: