Any time you transmit/transfer/send power (through wires in this case), not only is your transmitted power not all received/used at the end, but the voltage is reduced as well. So you have both a power loss and a voltage loss.
In order to transfer a given amount of power (let's say 100W), you can use any combination of voltage and current to get that much power, for example 10V at 10A, or 100V at 1A - the power is 100W in both cases. But the power delivered in the end is not 100W, and neither is the voltage the same at the end.
Here is a practical example: let's say you want to use the existing wiring in your house for the lights and use 10-volt bulbs instead of 100-volt bulbs, and let's say the wire is AWG 16 or around 1.3mm² and about 20 yards or 20 meters long from the power source to the bulb.
At 100V, the current will be 1A, the voltage drop on both wires will be around 0.53V (0.53% voltage drop), and the voltage across the bulb will be around 99.47V, which is unnoticeable to either you or your bulb (the bulb will be using almost 100W).
Reduce the voltage down to 10V, the current will have to be 10A for the same amount of power (100W), and now the voltage drop across the wires will be around 5.3V (53% or more than half of the voltage is dropped across the wires). The bulb will only have 4.7V across it and will use only 47W of the 100W sent from the power supply, which is definitely noticeable. The wires would get slightly warmer from the wasted heat and drop even slightly more voltage and power across them.
If you wanted to have the full 10V and 100W at the bulb, you would have to raise the power supply voltage to 15.3V (to make up for the voltage drop in the wires), but you would still be wasting the same amount of power, and you would have to supply 153W instead of 100W.
You could use much thicker wires, but they cost much more and are harder to work with. It would sometimes be hard to even fit them in the walls and other spaces.
P.S.: I have used 10V and 100V for the ease of calculation and understanding, and rounded some numbers, but it makes the point.