When we manufacture any product, usually we manufacture it by casting or forging process. To get a proper surface finish of that product we need a proper machining. At the time of maching a certain amount of material usually removed from the surface of the core product. For this reason a tolerance is needed over the designed dimension to maintain the actual dimension. Beside this reason, tolerances also kept to maintain the needed dimension after thermal expansion or contraction at the time of solidification during casting process.

generally tolerence are required for to control the quality of the product during mfg.

GD & T is coontroll the dimension of the product during manufacturing .it mean it give the perticuler range of geometric parameters like surface finish,flatness,strigtness,...etc...,

sometime tolerence are given for the purpose of the type of fit requirement between two meting parts,

sometime tolerence stack up analysis is also required when more than 2 parts are assembled to each other..

This is an easy to understand pdf on the topic. http://www.pages.drexel.edu/~rcc34/Files/Teaching/MEM201%20L6-Tolerance_RC.pdf

In addition, tolerance is required due to the material reaction after it is subjected under extreme conditions of heat, moisture or the environment. 

Tolerances are allowed so it aid in interchangeability of the product during use, assembly and when subjected to different conditions. It is most derived from the purpose of which the product is to be used for. Environment i.e temperature , application depending on speeds,  assembly so it can suit global use. 

What is the relation between tolerane and machining parameters?