First of all keeping other lines open provides a means for the railroad to serve additional areas on a regular basis.

Transporting goods by train is more economical that by truck by the amount of fuel that is used per movement. Also, by keeping rail lines open it provides a means to reduce gridlock on the roadways by providing an alternative means to ship goods which results in less fuel use and perhaps less congestion on the roads that would slow the transport of those goods.

And another point, as world population increases the need for additional rail transportation will increase likewise because of the increase need to ship goods and provide services so keeping alternative rail lines open an in regular use can help reduce gridlock of rail traffic on certain rail lines by providing additional capacity (rail lines) for the additional rail traffic to move on.

Many rail lines have been abandoned that would not be useless if they were still in use today such as the Vacaville to Esparto line of the Old SP that hauled onions and apricots in which there is still a market for today. The apricot wherehouse is still active in Winters were the old rail line used to be for example.