Even clean water left long enough on clean tiles will collect dirt from the surrounding air. As it dries up, it leaves a stain on the wall. The longer the stain is left there, the more dirt it accumulates and the more stubbornly it sticks there. But such a stain can still be washed away with a good scrub, a spray of clean water and a dry cloth to wipe it all off.
The above observation somewhat synch with the fact that like us, our Buddha-nature is pure and undefiled. But after staying in Samsara for too long, we eventually got affected by our surroundings. And the longer we stay attached, the more contaminated we tend to get. Mindfulness of the Buddha is like a good rub, and the blessings we get from the Buddha is like the clean water for a wash. Once we enter Pure Land, it is like the dry cloth that wipe us clear of remnant samsaric dirt.