SID's and STAR's exist primarily to cut down on frequency congestion and pilot/controller workload. If everyone is playing from the same sheet of music and knows what is going to happen, then it makes life simpler for everyone involved.
Example of a clearence without SID's and STAR's from memphis to st. louis:
KMEM direct MAW direct CGI CGI 335 degree radial to QBALL then STL 156 degree Radial to CHIKN direct KSTL.
Example of a clearence with SID's and STAR's (same route):
Memphis 5 depature MAW QBALL 3 arrival KSTL
See the differance? Lots easier for both parties involved.
The FAA decides which airports will have SID's and STAR's, then after a period of industry review they are put into use.
Oh yeah.. almost forgot! SID's are called DP's now (Departure Procedures). Old habits die hard.