Supporting the mechanisms that move on the robot is very important. Without planning proper supports, shafts may not be able to spin and your mechanisms or actuators could be damaged.
Forces, or "loads", that are at a right angle, or "normal" to the shaft, are the most important forces to counteract. The floor pushing on a wheel or two gears pushing against each other are two examples of normal forces.
A shaft should be supported with two points of contact. Without two support points, the shaft can pivot in the direction of the force. Ideally, those two points of contact should "capture" the mechanism under load. In other words, the support points are on either side of the mechanism. If a mechanism can't be captured, it is important to keep the load as close to the two support points as possible. Below are some examples of three major supported load configurations: Captured, Near, and Far
Additionally, the further apart the two support points are from each other, the better it can resist effects of a normal force. As the supports move closer together, they begin to act more like a single support. Supporting a shaft is important, but adding more than two support points can have diminishing returns. Each bearing that the shaft passes through adds a constraint to that shaft. You need to balance having the appropriate amount of constraints to keep the shaft from moving due to normal forces, but not too many that the shaft becomes “overconstrained.” Overconstrained mechanisms can bind and make rotation difficult, causing stress on the actuators and even damaging components of your robot.
The diagram below gives an overview of how bearing quantity and arrangement can impact the stability of load configuration.