London Midland are always doing this on their Euston to Crewe service. If it is delayed by about 30 minutes then will it have it run non stop between Stafford and Crewe direct via the WCML rather then via Stoke. Train still ends up having quite a dwell time at Crewe before the return leg back to Euston so I think its to massage the stastics.
I notice Merseyrail "sometimes" do this in the event of a late arrival into Southport from Hunts Cross and it has missed its booked departure time back to Hunts Cross. They will have the train run semi fast from Southport calling at (Birkdale), Formby, Waterloo, Sandhills, (Moorfields), Liverpool Central and all stations to Hunts Cross. (Stations in brackets) - it depends on the day, sometimes these are included in the semi fast pattern, sometimes not. Normally the train will get to Waterloo roughtly on time and Sandhills a couple of minutes compared to the time you should have arrived. However, no body bothers overriding the automatic signal system and the train ends up coming to nearly a hault at Blundellsands with a cauton while the level crossing is going down. A bit of human intervention needed.
These sort of practices can be very confusing if you do not know the geography of the area.