They may not stretch,... but it is a possibility. There are two reasons to use new bolts:
1) the original bolt is in a high load situation but is not a "torque to yield" bolt. Normally, these will never give you any problems but it is common place to replace bolts in a very high load application with new parts, just in case.
2) the bolt is a torque to yield bolt. This means that after an initial torque is applied, another torque usually consisting of a degree of movement rather than a measured torque, is applied to the bolt. The elasticity of the bolt then is used to apply an added degree of torque to the given part. Examples of torque to yield bolts are Head bolts, some connecting rod bolts, bell housing bolts, flywheel bolts.
Hope that gives you some insight.