As far as I understood, this locks should be only active as long as the “saving” process of the files take, so just something in seconds dimension. Because it is by default done via database, it is not too efficient and the unlocking can fail due to database overload and stuff. Excuse me, if the technical details might be explained rough/wrong 
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The cron jobs task will be only to release those not sucessful unlocked files. In older versions of Nextcloud this was not done and you can find several topics here about permanent locked files and guides how to unlock those manually via database. Now the cronjob resolved that regularly.
To use redis for file locking via memory caching enhances performance of this task signifikantly and takes it/the load from database. Thus this missed releases should not occur anymore and the cron job, which does jusk scan database, is obsolete.
Hope I explained everything right enough 
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