Modern computers are extremely reliable. In most cases, they work consistently over very long periods because the hardware components inside are manufactured to very high standards.
As with all electronic devices, faults can occur but when they do our consumer legislation provides an excellent level of protection.
Software issues are not covered by after-sales care packages, but forums like ours can usually help.
Most consumers will never need to take advantage of an after-care system, so it's a matter of personal judgement as to whether or not you pay for one.