Disk storage is cheap. It really is very cheap. It's so cheap it is not even worth sorting your data to delete stuff you don't want any more. So that's what most people do, just letting the old data build up. The trouble is, once you accept that you can't afford to lose it, it starts to become really quite expensive.
Let's say we store 160 gigabytes of data on a standard Serial ATA disk, the type that comes in any PC. That will cost around £55 or less, say around £0.35 a GB. But SATA disks are low cost and not intended for hard work in a server. A 300GB SCSI or SAS drive for a server will cost more like £400 or around £1.35 a GB, four times as much (these figures are just approximate and vary greatly for corporate buying, but the ratio and the principle remains the same).
If you could just keep adding disks then this would still be cheap. But of course you need something to read the disks with. If you fully cost the storage in a PC you might end up at around £1.20 a GB.
But this storage is not really suitable for sharing; a bit hard to manage as it is scattered all over the place; and vulnerable to hardware failure or theft. So you need a server to manage the storage.
With shared storage you need to use a RAID array in case an individual disk fails. You also need the operating system on one set of disks and the data on another. You need a reasonable server to put it all in, a rack to put the server in, an Uninterruptible Power Supply in case the mains fail. This will cost around £4.25 a GB.
So adding cheap extra hard disk space to a PC you already own costs you £0.35. But fully costed storage on a server costs you £4.25, 12 times as much. And we have not even backed it up yet. Or paid the staff to set it up.
Business storage is still relatively expensive and you can't afford to waste it on data you don't really need. On the other hand you can't afford the time and effort to go through it all.
Archiving unused data is the solution to preserving data without paying for expensive storage. A good storage management system will sift through the data and park the less used data onto a cheaper form of storage. In a small company with one server you could put the data onto a PC with large disks. In a larger company you could have one server with a very large array of cheaper disks for the archive. The software can identify data that has not changed in the last six months, and move it to different storage with read-only access. One backup can be made, and not made again as the data can not change. Any data that needs to be changed can be retrieved and put back on the expensive storage. In another six months you make another archive and back it up once.
If half of your data is unused in the last six months, and if the archive storage costs around a quarter as much as the normal, then you will reduce your storage costs by nearly 40%. That's not bad for a day's work.