The inclusion of a harp in an orchestra depends upon a couple different things: the type of orchestra and the music composition.
First, not all orchestras require harps. (On a side note, a harp is a string instrument.) In a classical orchestra, a harp is not used. Instead, the string section of the classic orchestra only includes the playing of violins, violas, violoncellos, and contra-basses. Also, the symphony orchestra does not use a harp unless the musical score calls for it specifically. An early Romantic orchestra does include the use of a harp (along with the following string instruments: violins, violas, violoncellos, and double basses). A late Romantic orchestra includes the use of two harps. This type of orchestra also includes the following sting instruments: violins, violas, violoncellos, and double basses. Lastly, a Modern orchestra includes the use of one to two harps and violins, violas, violoncellos, and double basses.
Outside of the orchestra's composition (according to type of orchestra) some musical compositions include the use of music for a harp. Here, a harp may be added to the orchestra in order to adhere to the composition itself. Typically, the harp is used in solos (written specifically for the harp) given the music of the harp is drowned out by the other instruments.
Therefore, the harp is required in orchestras when the orchestra itself requires the music of the harp.