Dust

I cover my Cary 211 FE amps primarily for protection from my cats but my VPI rim drive TNT turntable I cover to keep the dust off. All covers were made by Stereo Squares.

Lived in Arizona for 25 years... never covered. Have not in the ensuing 20 years. Never had any problems because of it.

It can be dusty, it just can't get humid. That is a killer for rust.. AZ is hot but dry.. Here we have due in the spring and fall. But set STUFF a barn around here it will never rust because the humidity stays low enough.. No salt on the roads. The sun is a killer though, AZ knows that.

Ruins everything, BUT can't live without it.. we hit 115 or 17 here last year.. BUT the over all year was cooler. Gets hot here, too.

Silks are very mold resistant to, I forgot that.. They can wick very well and still stay pretty mold and spore resilient. The spores don't stick as easy..

Mechanical items like turntables and Tape decks should be covered. It is not only dust you are protecting them from but other pollution like cooking fumes. Purely electronic devices could care less. I just wipe them off with a damp cloth once in a while. For units with fans it is a good idea to blow them out with compressed air once in a while. 

Actually, heatsinks and other components that run hot are impacted by dust which is like a wool coat to a transistor.  Not a good thing.  I have always covered my gear with DIY black synthetic fleece covers.