I know that a double pole switch is used to cut off the current in both live and neutral wire simultaneously, but by using a single pole switch,if you switch off the current through the live wire, the current doesn't flow through the neutral wire as it is. Then why can't a single pole switch be used in place of double pole switch?
On-off vs isolation switching
In the UK, a single-pole switch is usually used to turn on and off a light or an appliance. A double-pole switch is used where you need to isolate a circuit for safety reasons during maintenance of the circuit.
For example, current UK code requires a two-pole isolator for fire-alarm control-boxes and for high-power fixed-wiring (non-plugged) appliances such as isolation switches next to electric cookers and electric showers.
The issue is that, in case of a fault being investigated, the fault may have caused neutral to be broken or otherwise to be carrying a voltage significantly different from ground (earth).
It really depends on your application and the risk. First, you are absolutely correct that severing one pole will shut off the appliance, however it will leave the appliance's internal parts "floating" at whatever the voltage is on the other wire. If that is neutral, that is safe enough, barring a neutral problem.
For instance most countries have a way of supplying two poles for a higher voltage (240V in the US and Philippines, 208V in NYC, 220V in Brazil, 400V in most of the rest of the world) neither of which are neutral. In those cases you want a 2-pole switch so the machine isn't left energized at the other voltage.
Another case is if there's a chance of neutral not being safe. In that case you would want the maintenance shutoff switch to shut off neutral too.
Another case is when disconnecting a device with a neutral-ground bond of its own, typically a generator. If you do not disconnect neutral, and there's a problem with neutral or ground in the panel, neutral could try returning via the generator's neutral wire, to the generator's N-G bond, back via the generator's ground wire, then back into the non-broken part of the main panel. That could flow a lot of current through probably-thin wires to the generator, and remember, neutrals and grounds do not have circuit breakers, nothing would stop this until it set the generator or house on fire. Even in normal times, this would play havoc with GFCI aka RCD units.
If the power tool is low amperage (3 or 4 amps) and double insulated (the tool came with single pole switch) it's not so bad to use a single pole switch on the hot/ load/black wire. (while the neutral/white wire is left connected straight through with no switch) But if the power tool is heavy amperage (5 to 10 AMPS) and has a metal body (that YOU TOUCH while it's running ie it's not double insulated) then it's safer to use a double pole switch. (If the tool came with a double pole switch) This all applies to 120v single phase type power tools. I have put a single pole switch in place of a double pole on a 9.5 amp 120 v chop saw. (temporarily and you have to wire it differently obviously) Later I put the double pole back. I think it's safer. Seems like sometimes I could feel a little voltage bleed tingle when I ran the single pole. (very rarely only every now and then)
A double-pole switch is not specifically designed to open contact on a hot and neutral, although it could be used for that. It is more often used to open contact on two hot wires simultaneously, such as for a 220 volt circuit that uses two hot conductors, or for two 110 volt circuits that each use a single hot conductor.
I am sure there are many other applications, and this is a simplistic explanation.