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In most of the world, if you want to travel from A to B, you buy a ticket from A to B and that's it.

However, in India, it's possible to buy a ticket from X to B, designate a Boarding Point in A somewhere along the way, and board the train at A. Makemytrip explains how this works:

What is a boarding point?

You can choose a boarding point, from where you will board the train.
This can be any station between the Reservation From station and
Reservation Upto station. By default, you Reservation From station is
also your boarding point.

While you may choose a boarding point as any station on the route of
the train, you would have to pay the fare as applicable for
Reservation From – Reservation To stations. For example, if you are
booking a ticket from Mumbai Central to New Delhi and choose the
boarding point as Kota Jn, then you can board the train only at Kota
Jn and not between Mumbai Central and Kota Jn. You would however have
to pay the fare for Mumbai Central to New Delhi

The question left unanswered is why would you want to do this? It would seem much more logical and cheaper to just buy a ticket from Kota Jn to New Delhi.

You can change boarding point any time you like after booking the ticket, so that you don't have to cancel the ticket and buy another.

Railway will cancel your ticket if they can't find you in a fixed number of stops and will assign your reserved seat to a person who boarded the train with waiting list ticket.

Also, some stations will have a fixed quota of tatkal tickets. If train runs from X-A-B, where A is a popular station where people are more likely to take the ticket and run out of quota, it makes sensible to take ticket from X-B and change boarding station to A, afterwards. (Source: My friend who is expert at ticketing)

In Indian Railways there is a system called Reservation Against Cancellation or RAC. This is, let us say there are 700 seats in a train, then 750 people can book the ticket and board that particular train. A few seats (one per compartment) will be given to two people instead of one. So basically two people are alotted to the same seat. Now let us say someone did not board the train. Then the conductor will allocate that seat to one of the person who is under RAC.

Now let us say, you booked a ticket from Mumbai, and decided to boared from Kota; the problem now is, when the conductor comes, you have not boarded from Mumbai and your seat will be allocated to the guy under RAC. To prevent this from happening you have to change your boarding point to Kota, so that the conductor knows that you didn't miss the train but will instead board from Kota, and your seat won't be allocated to someone else.