why do people feel the need to buy expensive cable

In order to get the benefit of good expensive cabels, the rest of the system must be equal good.I think you can only get the full potential of your equipment when you use eg. equal good cabels. The same as with decoupling. The weakest link in the chain sets the level and cabels are also a component.
At a certain level of refinement, it becomes much more obvious what changes in the system, does to the sound. It is possibel to get a concert into your lisiningroom but everything in your system must be in ballance and at a very high level.
Better cabels helps the system haveing a open, airy, natural clear 3D view without artifacts at all.
Its like looking at a 300" widescreen, its fantastic but the flaws are easily seen, therefore everything must be perfect or you better stay with the 50" screen and save the expensive cabels.
Getting an ultra highend system to play correct and find the full potential takes years and thats the art about it. The closer you get, the more you fell the transformation of feelings from the musicians.

10-26-15: Tonykay
" ... If you have the means, go for it. But I don't believe you are getting your money's worth."

What we have here is the enduring law of diminishing returns. It's inescapable. Each person must settle the value equation for himself.

Because they have big investments in their systems and are concerned the wires would be the bottleneck otherwise.

The resolution capabilities of your system will directly impact the impact (good or bad) of the cables in your audio emjoymemt.
That elusive ethereal positive impact us an alchemy derived from the synergy (or lack of it) from ALL of ICs, speaker and power cables, layered on to the synergy with your electronics and your speakers.

High Price -- in isolation to the above -- is a red herring other than quality build product costs .... Full stop.

One size does not fit all - I've heard high end cables that sound great in system A but swapped into system B, they sounded like sh*t.

I buy and enjoy used components that hold their value for years. Key is buying from a mainstream manufacturer with a big (cult?) following, good support and longevity. For me that is ARC, VPI, Nordost and Vandersteen. In that light, cables are as much an investment as is an amp. Buying at half price I'm not going to loose much. The demand will always be there. Currently I'm running a tweaked VPI HW-19 MKIV/JMW with Ortofon Cadenza Bronze, ARC PH7, ARC REF3, Ayre V-5xe and Vandersteen 3A Sigs. They are strung together with Nordost Frey2 interconnects and a Acoustic Zen Satori Shotgun SC's. An engaging system. Recently I was Blown Away by replacing the IC between the pre and main amps with a Nordost Tyr 2. That IC breathed a life and magic into my rig that is the Holy Grail we all hope for. It is without reservation the biggest leap in system satisfaction I have ever experienced. In addition to the Frey 2, I have lower priced cables around the house by Kimber, LAD, Audioquest and Zu. They can't touch the Frey 2 by a long shot. The Frey 2 can't touch the Tyr 2 by along shot. On a (much) lesser system all these cables sound a lot alike and I would not bother with more than a very basic entry cable by Audioquest. It's not worth it.