Bearing spacers. Never really understood them.
I use spacers to this day.
Bearing Care 101:
A spacer that is the correct width (fits snug on the inner diameter of your wheel between the bearings) takes a lot of stress off of the bearing. Once the wheel is mounted on the axle the spacers line up with the hard inner ring on the bearing, essentially creating a metal tube all of the way through the wheel. This stabilizes the polyurethane wheel, which can be flexible under pressure.
I also have a thin speed ring (washer) on both sides of the wheel, also lining up with the inner ring, obviously to prevent the bearing from touching the axle and nut. This is another very important detail Hold 4 spacers and 8 speed rings in your hand, it weighs almost nothing but helps you go faster, your bearings don't wear as fast, and they very rarely break. For something that doesn't add noticeable weight and can't be seen once put together, there's no reason not to have them. Do a couple other things like keep your bearings clean and use the correct lube, and you will notice a difference.
About the lube, spraying WD40 on them is bad. It's greasy, collects dirt, and the chemicals in it are bad for the urethane. Use a greaseless lubricant like 3 in 1 or one of the light bearing oils you should be able to find at any skate shop. Again though, dumping a bunch of this on the outer shield while your wheel is on isn't the proper way to lube your bearings. this mostly oils up the space between the bearing/axle and bearing/nut. Not necessary because there should be a washer there anyway. Doing it that way and a little might seep into the moving parts but if you want to oil your bearings take them out, clean them thoroughly with a thick and dry paper towel (blue shop rags work great), and remove one shield. Apply 1 drop to the outer track and a drop to the inner track, snap the shield back on, you're done. If you don't have removable shields then lay the bearings flat, put a few drops around the shield, let it sit for 15 minutes, turn over and do the same thing to the other side. Dab the outsides to remove excess lube, put your shit back together and go skate.
So yes, spacers have a valid function. However, if your spacers are shorter than the space in your wheel between the bearings then they're pretty pointless.