Virtual Tape Library (VTL) is software that completely emulates a standard library, the introduction of virtual tape is seamless and transparent to existing tape backup/recovery applications. It uses disk space and represents it as Tape Media. It provides the ability to have the performance of disk, but look like Tapes. It stores data on the disk only.
A VTL solution can perform 10-times tape speeds for backups. Speeding up the backups will greatly shrink the backup window, which allows servers to be backed up faster. With existing backups finishing quicker, second- and third-tier servers that have not been backed up in the past may now fit into the backup schedule.
They are particularly good if your clients can only produce low throughput of data, as they do not suffer from 'shoe-shining' which damages real Tape drives and Tapes when they are run too slowly. You can then duplicate the images to Tape; this will usually run the real drives at a good speed, as you are coming from dedicated disk. However, you can achieve this just using a basic disk/ advanced disk storage units (but they don't look like tapes).
However, disk backups are much reliable because your VTL software may be prone to corruption. Also, Backups should never only be run to disk, even if the disk (or VTL) replicates itself to another disk (or VTL) device outside of backup software - this does not prevent against corruption. If the disk is corrupted, this could be copied across and you could lose everything. If you duplicate to tape, then only a small amount of data might be lost, as the tapes are separate.
Tape drives are historically considered reliable, provided that they are correctly implemented into a backup environment and correctly looked after along with tapes. The only comparative problem with real Tape drives is that they have to be run above a certain speed to stream, and on average, many people do not do this. LTO drives are common, and have to stream else they stop/start which wreaks them, and the tapes. However, certain other drives (eg STK drives) are designed to stop/start and have no such limitations - so, designing a backup environment is about using the right type of drives, and many people use LTO drives when they are unsuitable, due to the limitations of the speed they can get the data off the clients.
To secure the backup taken on a VTL the Backup can be duplicated to real Tapes and retained for a longer duration.