Ignoring advance fares, which, as several posters have pointed out, have special restrictions, there are lots of fares where buying a ticket from A to C via B, (and only using it as far as B) is cheaper than buying a ticket from A to B - and it's perfectly legitimate.

Example: I want an off-peak day return ticket Birmingham - Crewe. Ask for this at the ticket office and you will be sold a saver at £20.90 - there is no day return for this flow.

However, ask for a day return to Chester - valid via Crewe - and it is only £15.60. You can use this as far as Crewe and no further, since break of journey is allowed in both directions. You can even buy a day return to Wrexham - valid via Chester and Crewe - for only £12.90, and again stop short at Crewe (and start your return journey there).