Hello @Stuart Newton
These table records data relate to cron. Basically, it is where the results of a cron task are stored. The daily cron email report aggregates its content from these rows.
At present it is not possible to prune this log from within WHMCS, however in WHMCS 8.1 and above we have optimized the indexing for cron log data.
CORE-14970 - Optimize indexing for cron log data
In the meantime, if you did want to do this, you could so with a SQL query.
For example the SQL query below will remove any entries in that table that predate 2020:
DELETE FROM `tbllog_register` WHERE created_at < "2020-01-01";
As always please backup your WHMCS DB before perfoming such operations.
I hope this information helps.