Why many passes at file removal? Why file removal tools offer gost 2 passes and British 3 passes or even gutman 35 passes options? In which case the single pseudo random pass is not enough?
This has to do with the physical nature of the storage device. Simply deleting the reference still leaves the data available for recovery. Writing once over randomly might still leave chunk traces of data which could be pieced together.

It is not generally considered easy to recover information overwritten once, and for your own pc... being donated or sold... once is probably enough. Depending on your own scale of paranoia you can use more passes.