You can delete the completed system jobs. You can just setup a recurring bulk delete job if you want to get rid of this records (Usually you'd setup a job to get rid of completed system jobs completed before a certain date, so you could keep some for any other miscellaneous purpose.
This table contains records for any asynchronous jobs that e.g. rollup fields calculations, asynchronous plugins, existing bulk delete jobs, essentially anything that runs asynchronously against that environment and is directly part of the environment e.g. workflows, fields, plugins etc
You can delete the environment as well if you wish and it should free up the capacity. I can't recall if there's a day delay for the space allocation to refresh in the admin portal