Pash - those introductory fares are part of the marketing tools at an airline's disposal (but which tend to be used by too few).
Let's look at two airlines, we'll call them A and B. A starts up a standard fares, full frills service, and does the usual advertising. "Fly with us from Geordieland to London for just £299 return!" trumpet the billboards and radio ads.
Well, wow, whoopee. If I have to fly urgently to London, I'll remember them. Possibly. Maybe. Unless I remember GNER first. Who I've used before and aren't too bad and OK, they take longer than flying but it's straight into Central London and ... what prompted this line of thought in the first place?
Airline A's initial flights are very lacklustre, with loadfactors averaging 10 - 20%. The pax are not wildly impressed, but hey, they're getting their Air Miles and the company's paying for it so who cares?
Airline B starts up its low fare operation, charging £10 from Geordieland to London. "Go with us for £10 each way!" the billboards trumpet.
Eeee, I must get online and book me ticket as soon as I get to a computer! Sheite, if I give me mates a call, we can all go down to London and see the match and have a brilliant time and it's a lot cheaper than that bloody GNER! Plus we all have to go and see the missus' family and they're all southern softies, so I'll book tickets for that as well at the same time...
Airline B's flights are 100% chokka. More importantly, word has got out about the company's services, so everyone has brand awareness. Even after fares are increased to more realistic levels, loads remain very high and Airline B adds extra frequencies. This attracts the pax who were flying Airline A who cease operating the route and claim that services between Geordieland and London are not economically viable.
Get the picture?