Why do we use a relay and switch to control on/off of horn circuit instead of using a simple swtich?

You really aren't understanding how a relay works or what it does. It states right in the quote that the control wire can only handle .25A and is very thin. The horn itself may require 24A or more.

When the relay is activated, it can transfer the needed amperage to the horn through wiring which is designed to take the load. There is only so much space within a steering wheel and column. In order to run wiring for everything which is controlled there, a relay is the way to go.

Consider in most vehicles you need to control things like the turn signals, lights (to include changing from low to high beam), ignition system, wipers/washers (front and back if so equipped), cruise control, etc. If every item was to be controlled directly, you'd have to increase the thickness of every wire and thus the size of the steering column to house it.

Besides this, you should also consider the cost to run such wires. You'd have to extend wiring to these specific devices of the larger size, which all costs more. It would also make the vehicle heavier.

Another thing to consider is the contact points in the horn button. In order to handle the load which would be put through the horn button, you'd have to build all points in it to handle the amperage if ran directly. A typical relay can run millions of cycles. To build the horn button to take the same amount of cycles, you'd have to have some serious engineering efforts going on. The relay not only makes the circuit, it also handles the load. The horn button itself is not designed the same way.

A relay is controlled by a switch or button or other electrical contact that causes an electric switch to make contact so that it now operates a device with a higher power demand although both are the same voltage.

Thinking out of the box for example only, now picture a sports field with flood lights and it is time to turn on the lights. Someone goes over to a simple wall switch, similar to those in your home, and flips it up. This causes a reaction in the relay, causing it to trip the bigger switches that actually turn on the field lights. Yes I know they are 2 different voltages but the actions are the same.

A relay isn't 'necessary' as such, it would operate just fine through the steering wheel switch in most cases, but a relay is designed in the circuit for many reasons.

Voltage drop - running a wire from the battery into the steering wheel and back out to the horn is a long path, and causes voltage drop from the resistance in the length of wire. Voltage drop may not be noticed in a horn, but will be noticed in sensitive components like head lights and spot lights.

Wire size - small wire is lighter, cheaper and takes up less space in the car.

Electrical switch size - relays allow cabin switches to be tiny, silent, easy to operate and last almost forever.