[1] The purpose of OTL is to set a limit on crews so that marshals and other personnel involved in running a rally only need to wait a fixed amount of time after the last car was due before closing the control.
Without OTL it could be very difficult to run a rally smoothly without having huge numbers of volunteers willing to wait around indefinitely.
For crews in a stage rally, OTL is a fixed limit of accrued lateness, after which one is no longer allowed to continue to compete.
In certain rallies (mainly road and navigational) it is also possible to make back lateness during the normal course of the event, subject to certain restrictions.
The official expansion of the term OTL is "over total lateness", although in everyday use only "over time limit" is generally used.