We examine plausible motivations for both using and building computational grids. We find two reasons to use such grids: the existence of a workload in which tasks have deadlines, but the load varies over time; and the existence of an upper limit on cost-effective parallel systems, forcing replication when greater degrees of parallelism are required. We speculate that there may be scope for public grids, in which protecting the integrity of information is not guaranteed, but that there is much larger potential for virtual private grids within organizations. In both cases, the form of markets, execution planning, and pricing is likely to be different from the frictionless markets predicted in the literature.