When you want to travel from London Euston to Manchester Piccadilly, you would expect to buy a ticket going directly from London Euston to Manchester Piccadilly. However, you may find it a lot cheaper to buy a ticket from London Euston to Stoke on Trent, and then one from Stoke on Trent to Manchester Piccadilly.
This means you could buy two independent tickets for the same journey, pay much less. The same applies to buying tickets more than two.
What’s more, you may even find it much cheaper if you accept flexible journey. For example, buy two tickets from London Euston to Stoke on Trent and from Stoke on Trent to Manchester Piccadilly, with 10 minutes wait at Stoke on Trent. These concepts are what we called
split journey tickets, and it helps passengers saving a lot money.