Why are spacecraft made to "spin" after launch? At some point after launch, usually just before or after separation from the last booster stage, spacecraft are often made to "spin" (about the axis of their trajectory)? See e.g this You Tube video. What is the reason for this spin? 
 A: Applying spin along an axis, usually the direction of travel, endows the craft with a degree of gyroscopic stability, enabling the craft to preserve consumables onboard as it makes navigation easier. SPin is also used in re-entry (like the beagle mars lander). 
New Horizons, Pioneer 1 and 2, Voyager 1 and 2, as well as some sattellites all use a spin to gain gyroscopic stability.
A: It's a great way to get gyroscopic stability.
NASA has been using this technique for a long time.  For instance, the Pioneer spacecraft used this method.  Another example is the Juno spacecraft as well.
I hope that answers your question sufficiently.
A: *

*Stabilization.  Example: Pioneer

*Equalize heating (barbecue mode).  Example: Apollo

*Deploy antennas & booms (via centripetal force).  Example: IMAGE

*Maintain tension in a solar sail.  Example: Cosmos 1 

*Test general relativity. Example:  LAGEOS

*Create artificial gravity.  Example: Gemini

*Simplify or reduce weight of sensors (e.g. star trackers).  Example: New Horizons

*Improve the accuracy of sensors.  Example: Hipparcos

*Keep antennas/sensors pointing towards a central body.  Example: GPS

A: Note that on liftoff some craft, such as the Space Shuttle, would spin a bit in order to point their UHF antennae at the ground station. You can see this in all the Shuttle launch videos as the shadows slide around a bit on the skin of the craft, and then stop.
