The design and installation of network cabling can be extremely complex. A whole host of factors have to be taken into account, network speed, length of cables, avoiding architectural features or electrical interference, and many more.
Many established networks contain a mixture of old and new cables, but which are Cat 4, Cat 5, or Cat5e. Sometimes when networks are upgraded older patch cables are left in place. Just because a cable was fine on the old network does not mean it will work on the new upgraded network. It can be hard to identify these faults without some kind of test kit.
Is the cable itself faulty? Checking the pairs will identify faults.
10M, 100M, Gigabit? Who knows? Something that reports network speed is very useful. Link testing can be used to identify the network speed. Its useful to find out additional network information using DHCP.
If you are running Gigabit on your network you need to ensure that your cabling is up to the Cat5e standard, the older Cat5 standard is fine for 100M but needs careful checking if you intend to run Gigabit. Many organizations only discover this too late.
The longer the cable the more the signal is reduced (attenuated), ultimately to the point where no signal gets through. Measuring cable lengths to check that they are within specification is important and remember that attenuation increases with network speed, so your new faster network may have cable lengths that were fine on your old network, but are now too long.
Are you sure that pin 1 is really connected to pin 1 at the other end? You need to check this to be certain. What about short circuits, crossed wires, split pairs etc.? Most of these faults can be detected using a continuity test, but some, like split pairs, require a signal to be sent down the wire. Something that can send a signal or tone is required.
You can also use tone generators for signal tracing and cable identification.
Ping tests confirm connectivity across the network, to PCs and Network Cards.
Voltage on the wire
Phantom voltages or electrical noise can cause faults on the network. Its useful to be able to check where voltage are occurring, either intentionally or not.
As a Network Manager you will be expected to deal with day to day cabling problems. You probably cant justify the cost of the specialist testers used by cabling professionals, but a simple testing kit will allow you to check for the most common problems, save you time chasing phantoms, and help you resolve issues faster and more efficiently.