1. Harper's New Monthly Magazine is a monthly magazine published from 1850-1913, and continued as Harper's Monthly Magazine from 1913-1976, and then simply Harper's from 1976 until today. It contains stories of general interest on a variety of subjects, for example, Volume 26 (Dec. 1862 - May 1863) contains articles about: Gas and Gas Making, Insects Injurious to Fruit, Cards and Dice, a Californian in Iceland, Victor Hugo in Exile, A Talk with Jefferson, the Home and the Flag, Some Seccesion Leaders, and also contained a monthly editor's section, fashion column, and current events summary.
In the early volumes, most of the articles are reprinted from other magazines, with some original content, but in later volumes, original content predominates.
Harper's also contains numerous illustrations, including photographs in the later editions.
Sort of the Reader's Digest of it's day.
1. Harper's has been published from 1850 until today under the three names listed above. It was published by Harper & Brothers of New York.
As for the why, in the words of the publisher "It was projected and commenced in the belief, that it might be made the means of bringing within the reach of the great mass of the American people, an immense amount of useful and entertaining reading matter," (Harper's Volume 1, introduction). I might also speculate that the publisher's were interested in making money in some capacity, especially with the later issues.
1. Who appears in it? The articles are written by a variety of writers, and are about a variety of figures. Who does not appear in it might be a better question.
2. Harper's is published in six month volumes with an index beginning each volume.
3. The only supplemental aids for finding articles are the Reader's Guide to Periodical Literature and the earlier Poole's Index to Periodical Literature. Beyond those two sources, one would have to scan the indexes that begin each volume.
4. Harper's can provide period perspectives on a variety of issues, but will not provide any in depth perspective, as it is more of a popular magazine. It would also provide access to contemporary literature that might otherwise be hard to find.