Have you ever tried to get an older keyboard work with a newer computer? Or how about trying to connect one old monitor to a newly purchased computer? Remember that old hard drive you couldn't hook up because it was made for IDE and you only had SATA available? Meshing older and newer technology has created an entire sub-industry, which has sprung up chiefly around computer adapters and connectors!
Sometimes, it's as simple as the need to change the male-female direction from an older monitor cable to a newer monitor port. Without the adapter, they are both female. With an adapter however, the computer's port can be given a male connector, allowing the older cable to plug into it. In reality, this scenario isn't always that simple! Sometimes it can be a real challenge to get the right adapter/connector combination that will successfully send the video signal to the older monitor.
Keyboard connectors have changed several times over the years as well. This has resulted in adapters needed to go from DB-9 to PS-2 or from PS-2 to USB. Taking a 9-pin mouse to PS-2 or USB ports also requires special adapters, and the 9-pin to USB is quite difficult to find. Printers have gone from two types of the ribbon-style connectors to USB ones.
These kinds of connectors, particularly going from one ribbon-style to another, are usually quite bulky compared to other connectors out there. Hard drive adapters now let you connect laptop hard drives to computers and older hard drives to newer ports. Also, USB hard drive cases can turn your old drive into a backup unit if you need one too!
Network adapters are interesting little devices that, if you talk to certain techs out there, supposedly do not exist. But these little units will let you extend the length of your network cable by allowing one Ethernet cable to plug into one end, and the other Ethernet cable to plug into the other end, without much detectable loss in network capacity. POE network adapters allow you to connect powered devices to your Ethernet cabled network, and be powered by the adapter itself, cutting down the need for power outlets. POE network adapters are actually network switches that usually let you plug in anywhere from 4 to 8 or even 16 devices, depending on the unit you buy.