Filters are designed to catch particles and trap them.
When new there are no particles, hence "clean", but still have some resistance to flow - in this case 70 kPa or 0.7 bar (quite high).
As the filter slowly clogs up the pressure drop increases up until it fails. Usually there are pressure alarms which tell the operator when he needs to change the filter or renew it / flush it.
There are many types of filter so it's not easy to say much more.
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