An analogy occurred to me - an altar could be compared to a telephone line. Whether it's an altar space to honour and petition your ancestor spirits, the saints or any other spirit, the altar functions like a communications device. The dustier, messier or more stagnant the altar the more interference one experiences on the line. It could get so bad that you can't connect with that spirit at all, or lines get crossed and you get snippets of distorted noise from other spirits you might not want to hear from.
More than that, cleansing your altar, dusting the icons replenishing the water, rearranging things just the way you know your spirits like them become an expression of your devotion and your connection to the spirits. You not only purify and rejuvenate the space, you subtly reaffirm your connection and intention to strengthen bonds with your spiritual world. You keep the line pure, open and free from interference.
After I'm done straightening things out, dusting, and wiping things down with bit of florida water I put some fresh stuff on the altar (water, flowers, offerings) and happily discover that there is a new buzz, a friendly hum - almost like that altar is idling, ready for more spiritual work.