At least one weekend in the year Network Rail have to carry out maintenance work on the heavily used East Coast Main Line which means rail journeys between Leeds and Kings Cross are disrupted. They don't do this work during the week which would inconvenience business travellers (because they pay the highest fares) but it is an alternative for travellers between the two cities. Regular railway maintenance is criticial if accidents are to be avoided. Remember when journey times from Leeds were extended to about four hours due to speed restrictions?
Gatwick is only 30 minutes by train to London Bridge and City Thameslink and 30 minutes to London Victoria. Its also useful for those travelling right down to the south coast. When the redevelopment of Blackfriars starts there will not be a through service from Kings Cross Thameslink to the South Coast.