The cable's effect not only differs for each kind of cable, but also its length & connectors, and the amp & speakers used. I'll keep this simple, since even that is above my pay grade, and only consider resistance, although changes in inductance & capacitance are also in play.
From the amp pov, the cable adds to the resistance it's driving. That usually results in a (slight!) reduction in the amp's output - but some circumstances could result in an increase. From the speaker side, the cable resistance adds to the source impedance, lowering damping & resulting in the amp having less control of the speaker (this is why tube amps generally have looser bass than ss - they tend to have a higher output impedance).
Sorry, it's hard not to say "impedance" when talking about audio - that's the result of the complex interaction of inductance (L), capacitance (C), & resistance (R) in the circuit and of course is frequency-dependent. So what I said above is true, but additionally complicated by the cable's L & C, and different at every frequency. The choice of amp & speakers also affects this interaction to varying degrees at every frequency.
Conclusion: If you like how your system sounds, good on you, mate! If not, cabling can make a difference, but the results are not as measurable or predictable as upgrading your electronics.