In today's world, I feel they have basically one purpose: BACKUP and RESTORE.
The capacities of each tape cassette vary greatly from about 680 MB to 4.4GB (8GB migration path) and more.
I would recommend a tape drive as a backup device for anyone with a network server, and for desktop PCs which have large hard drives. The cost is not prohibitive, $200 to 600.00., and the actual cartridge/cassettes are only a few dollars ($25.00 to 35.00). It is a cost effective method of backing up your system, but it is very slow.
One of the problems you may encounter is that many current tape drives have drivers for DOS, Windows95, but do not work with Windows NT due to lack of drivers.
An alternative might be a IOMEGA JAZ drive (1 GB per cartridge) or a SYJET (1.5GB per cartridge) - they are faster, but their capacity per cartridge is lower; and their purpose was not a backup device. The cost of JAZ or SyJET cartridges are close to $75 - $110.00 per cartridge. Very expensive to keep multiple backups when compared to a tape backup. Three to 4 times as expensive as tape!
Manufacturers:
Some individual state that removable hard drives can be used for backup. Yes - they can. But backups should be stored off site. They may be nearly as cheap as JAZ or SyJet cartridges, but they are bulkier, heavier, and much more inconvienent to carry around. Let your pocket book and your judgment be your guide!