Routinely taking grease or oil samples from gearboxes, bearings, pumps etc. allows their condition to be monitored with minimal effort. Any wear leads to an increased concentration of metallic particles even before macroscopic damages are visible. Any contamination, including moisture, is also detectable.
Additionally, lubricants age during use which means that their properties change over time and their functionality deteriorates as a result. This ageing process can also be observed and quantified through lubricant analysis, which can allow oil changes to be performed only when necessary. This point is especially important when considering minimizing costs and potential environmental impacts.