When should I use an Orb of Scouring?
I've used Orbs of Scouring a couple times so far. The main use of it seems to be if, for example you find a magic item with great sockets, but, still, it's only magic. You can use an Orb of Scouring to remove the magic affixes and return it to normal, then an Orb of Alchemy to turn it into a rare item, making it much more powerful and useful than it originally was.
Whetstones/Scraps are also much more effective on normal items than on magic/rare items, so it is oftentimes worth it to Scour an item back to normal, increase the quality, then use an Orb to return it to its former level. Of course, you won't want to do this if it already has excellent affixes, as the odds of getting them back will be almost nil.
One of the most common uses of Orb of Scouring is to lower the level of a gem by one level. This can be done by selling both the Orb and the gem to a shop.
The main point of these orbs is crafting. There are two parts to crafting, the first being regular crafting, the second one being chancing. There's technically a third option (repeatedly using chaos orbs on a rare item until it hits good rolls, known as chaos spamming), but I don't really see this as crafting. *
Regular crafting (expensive, albeit very profitable)
First thing you need for regular crafting is a good itembase. A good itembase is an item that has high basestats, for example the "Vaal Regalia", which currently has the highest base Energy Shield out of all body armor bases in the game. It's important to ensure that the itemlevel of the itembase is 84 or higher, due to tier restrictions.
The second thing you need is currency, and a lot of it. Being specific, you need the following currency items:
- Orb of Transmutation
- Orb of Augmentation
- Orb of Alteration
- Orb of Scouring
- Regal Orb
- Exalted Orb
What you do in the process of crafting is this:
- Use a transmute on your itembase
- Use alterations until you hit high rolled stats that you want (i.e. tier 1 flat Energy Shield)
- (Use an Orb of Augmentation if you only have one stat)
- (Reroll if the second stat is not desirable)
- Use a regal Orb to turn the item into a rare, adding another affix
- (Use an exalted Orb if you have 3 high-rolled (Tier 1/Tier 2) desirable affixes)
- (Use an Orb of Scouring if you don't)
- Rinse and repeat
Chancing
A bit of background story (as to why one'd do this) first:
Path of Exile features quite a lot of unique items. A few of those are actually unusable, some are too common to bear any value, and some are rather uncommon but not that desirable. Some, however, are extremely rare, and extremely desirable, making them extremely expensive.
A common example for this is the "Headhunter, Leather Belt". This item was only obtainable during the "Nemesis-league", and can no longer be obtained in the normal game except for Maps that have the Nemesis-Modifier, which can be activated by paying Zana a fee of a few chaos per map.
Note: As of now, the legacy league is taking place. During this league Zana does NOT offer any league-modifiers for map, as those can be obtained by using leaguestones. This means that you can chance a Headhunter while being in an area like "The Twilight Strand" while having the Nemesis-Leaguestone active.
The Headhunter is an incredibly strong item, and can be used in most builds. It's also quite fun to play with.
Now, when you go into a map with the Nemesis-Modifier, you can only take as much leather belts with you as your inventory allows you to, and you only have 6 portals available, which disappear once used. That's why quite a lot of people nowadays take around 8 leather belts, the itembase for the headhunter, with them, as well as a lot of chance & scouring orbs.
This allows them to enter the map, chance all the normal leather belts, and then to scour them in order to be able to chance them again (as chance orbs only work on normal items).
Short version: Orbs of Scouring are nowadays mostly used to clean itembases, intending on using them again, either to continue crafting them, or to continue chancing them.