Comment by roryirvine

Most likely because it's not just a single batch job, but a whole series which have been scheduled based on a rough estimate of how long the jobs around them will take.

For example, imagine it's 1997 and you're creating a job which produces a summary report based on the number of total number of cars registered, grouped by manufacturer and model.

Licensed car dealers can submit updates to the list of available models by uploading an EDIFACT file using FTP or AS1. Those uploads are processed nightly by a job which runs at 0247. You check the logs for the past year, and find that this usually takes less than 5 minutes to run, but has on two occasions taken closer to 20 minutes.

Since you want to have the updated list of models available before you run your summary job, you therefore schedule it to run at 0312 - leaving a gap of 25 minutes just in case. You document your reasoning as a comment in the production control file used to schedule this sequence of jobs.

Ten years later, and manufacturers can now upload using SFTP or AS2, and you start thinking about ditching EDIFACT altogether and providing a SOAP interface instead. In another ten years you switch off the FTP facility, but still accept EDIFACT uploads via AS2 as a courtesy to the one dealership that still does that.

Another eight years have passed. The job which ingests the updated model data is now a no-op and reliably runs in less than a millisecond every night. But your summary report is still scheduled for 0312.

And there might well be tens of thousands of jobs, each with hundreds of dependencies. Altering that schedule is going to be a major piece of work in itself.