Watch the video to find out and make your bike better!

To start, what is a bottom bracket bearing composed of?
It is an, Inner race, steel balls and an outer race.
The outer race is what touches the frame, the inner race is what goes around the spindle. Its worth mentioning too that the bearing is only meant to spin around one axis.

Since the bearings can only be pushed into the frame so far before the outer race is stopped, having no spacer means that tightening the cranks pushes the inner bearing races toward the center of the frame and each other.

Basically the outer race cannot move so the force on the inner race is constantly trying to separate the bearing.
This is called side loading.

As bearings start to wear, the tolerances between steel balls and the races grow, allowing them to move independently of each other. If there is any movement between the bearing parts you will most likely hear it when you pedal.

On top of this, having no spacer allows pedaling to put other forces on the bearings.

Each rotation of the cranks puts both forward and backward force on each bearing during the entire rotation and tries to make the inner bearing races wobble off axis. In general this makes for unnecessary drag on the bearings when pedaling.

With cranks getting beefier and spindles growing, bearing size has had to shrink and there is less flex in the cranks or spindle. Less flex equals more stress on these smaller bearings when pedaling. Unless you have a bottom bracket spacer between the bearings.

Bottom bracket spacers remove the different forces on the bearings by placing something between the inner bearing races. This means that when the cranks are tightened, there is something to tighten against. Rather than constantly pushing the inner races toward each other with nothing to tighten against, the force is distributed evenly, and the bearings are not as stressed by tightening or pedaling.

Imagine it like this, if you have just an axle in your fork dropouts with no hub and you tighten axle nuts against the dropouts, the dropouts would have nothing to tighten against and would bend toward each other. Add in the hub and that empty space is filled, giving the dropouts something to tighten against.

So not only will having a spacer eliminate side load and quiet your creaky bottom bracket, it will also make for a smoother pedaling experience.

Another thing this can be good for is the ability to have tight cranks and still be able to crank flip.
With the inner bearing races in alignment with the outer races and tight against something on both sides, the tightness of the cranks doesn’t have an affect on how they spin.
So if you’ve ever wondered why your cranks wont spin with a loose chain and without loosening your cranks, this is probably the reason.

Some frames have different width bottom bracket shells that will need different spacers, however if you buy a complete bottom bracket set up it should come with everything you need for the different width shells.
Common widths are 68 and 73mm.
My profile bottom bracket came with both a 68mm spacer as well as a 5mm spacer to make 73mm.

To know what width your bottom bracket is, simply measure the tube from end to end as shown.
Another side note, Odyssey has a great write up from 2004 about this topic on their site that I will link below.

So what do you think?
Are bottom bracket spacers important? Is it worth the trouble of taking out your bearings? Or do you not care?