Spacers, often referred to as bearing spacers, are sometimes installed in skateboard bearing setups to increase the life and performance of your bearings. A spacer's primary purpose is to stabilize the bearing, reducing excess movement on sharp turns and curves. While certain boards use spacers, others do not as a sign of craftsmanship and the quality of that board's bearings.
If you are an aggressive skater who makes sudden stops and puts extreme weight on either side of your skateboard, it might be in your best interest to install spacers or buy a skateboard with bearing spacers installed. The main purpose of bearing spacers is to maintain alignment while riding your board. If installed, bearing spacers can help increase the life of your bearings, allowing you to skate for longer without purchasing or replacing your spacers. If you skate aggressively without spacers, you run the risk of damaging your bearings during kick flips and sharp turns.
If you are a casual skater who enjoys riding longboards and other boards that do not require sharp turning, bearing spacers are not necessary. Many longboards come without bearing spacers, opting instead to securely tighten the regular bearings into the wheels of the longboards themselves. Not installing spacers will save you time and money and reduce your chances of unnecessarily harming the bearings in your skateboard. In addition, spacers can sometimes prevent your bearings from rotating with your wheels properly, especially if you have installed spacers that do not perfectly match up with the size of your bearings.
A popular argument for why spacers are not necessary is based around the natural corrosive properties of ball bearings. Ball bearings in your skateboard have finite lifespan that is shortened by corrosion. Corrosion comes from external elements, such as dirt and rock that gets integrated into your bearings while skating. While bearing spacers might increase the life of your bearings, the argument is that your bearings will wear out from corrosion long before the bearings break down because of a lack of spacers.
While spacers for your skateboard can potentially help stabilize and increase the life of your bearings, be careful when installing anything new on your skateboard. If you do not feel comfortable installing spacers or your bearings, take the board to a licensed skate shop to ensure quality and safety. Failure to do so could result in instability with respect to your trucks, bearings and even your deck itself.