Contamination of other worlds is a major source of concern, particularly when we’re going someplace that might be habitable for some form of life. All spacecraft (and this includes satellites, telescopes, and rovers) have all of their pieces go through some pretty serious decontamination before launch, and most are assembled in a very high test clean room. (The last time I was near a clean room of this caliber, we weren’t allowed in the corridor that eventually led to the clean room, to try and keep airborne particles to a minimum).
Part of this clean assembly is for the good of the craft itself. Discovering that somehow, dust settled on the mirror of your brand new telescope would be a nightmare scenario; this means that your telescope is seriously underperforming, to the detriment of all the science that it might have been able to do. And you certainly don’t want grit making its way into the moving parts, which could jam the operation of those parts. If the part that needs to move is critical to the success of the mission (say, for instance, it’s your communications antenna), a stuck part could cripple the entire mission. So keeping everything as clean as possible gives the craft itself the best possible chance at operating the way it was intended.
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