Assume that we have a generator or a power plant that generates electrical energy to power up an appliance located far away. I'll make these assumptions:
The working voltage required by the appliance: 100 V
Distance between the appliance and the power plant: 1 km
Transmission wire resistance: 1 ohm
Appliance load: 10,000 watts (if the load is resistive, it means the load resistance is 1 ohm for the rated voltage of 100 V)
I am not fixing the voltage that the power plant can generate. Deliberately keeping it flexible to simplify things.
We don't use a step-up transformer. The load will require 10000/100 amps = 100 amps of current. Voltage drop in transmission line = 100 amps x 1 ohm = 100 V.
Thus power plant needs to generate 200 V and 100 amps to deliver full power to the load.
Wasted power in transmission line = 100 A x 100 A x 1 ohm = 10,000 watts. Thus almost half the power gets wasted in the transmission line itself. I deliver 10,000 watts to the load and waste 10,000 watts in the transmission wires.
If we assume that the power plant can only generate 100 V. Then we can't even supply full working voltage to the load because of transmission voltage drop.
Assuming 100 V as power plant voltage and the load being a resistive load (Rload = 1 ohm), the situation becomes somewhat like this:
Total circuit resistance (load + transmission lines) = 2 ohms
Current = 100V/2ohms = 50 amps
Power wasted in transmission lines = 5,000 watts.
Power delivered to the load = 5,000 watts.
Thus we can't deliver the full power to the load. Also, my load will get only 50 V instead of 100 V. In practice, many loads are non-resistive and the way household appliances are made, it might not even turn ON at half the working voltage.
Let's have a step-up transformer before transmission lines and a step-down after the transmission line. Let's generate 100 V from the power generator and step up this voltage 100 times.
On the input side near the power plant, we feed 100 V and get 10000 V out. Current at this voltage = 10000 / 10000 A = 1 A.
I just need to send 1 A through my 1 km long transmission cable. Voltage drop = 1 x 1 = 1 V which is negligible.
Power loss in the transmission line = 1 x 1 x 1 = 1 W.
I will be able to step this voltage down near the load using a step-down transformer and feed to the load. My load gets full power (10,000 W) while I lose only 1 W in the transmission lines.