Tournament chess is played under a strict time control, and a program must make decisions about how much time to use for each move. Most chess programs do not set out to search to a fixed depth, but use a technique called iterative deepening. This means a program does a depth two search, then a depth three search, then a depth four search, and so on until the allotted time has run out. When the time is up, the program returns its current best guess at the move to make.
Iterative deepening has the additional advantage that it facilitates move ordering. The program knows which move was best at the previous level of iterative deepening, and it searches this principal variation first at each new level. The extra time spent searching early levels is more than repaid by the gain due to accurate move ordering.