It's an acid that chemically breaks down the fibers in the cloths, this makes them softer. Some like the feel of it. However, it also weakens the fabric, causing the clothes ware out faster.

If you like the extra soft clothes, and don't mind replacing them often, it's fine.
If your clothes are rather expensive, and you want to keep them as long as possible, it'd be better avoid Fabric Softener/Conditioner. In fact use cold water, and Woolite Dark detergent (or whatever local equivalent you have) for maximum longevity of your cloths, color white or other.

I got that advice from a metal music fan site, offering care instructions for collectable tour shirts that you want to ensure remain a black as your soul. They actually suggested using water form a Nordic iceberg, since that's where all the blackest metal comes from anyway. It was very funny. I'll try to see if it's still online.

Edit: I miss remembered much of it, like the Nordic Ice. And the original appears to be gone. But someone else has clipped the important bit.

… for you, the truly elite metalheads, I am here to bestow the darkest secrets of Laundromancy upon thee. I have consulted tomes of ancient wisdom (i.e., I asked my mom) in preparing this guide. May it serve you well… in darkness:

1. WOOLITE IST KRIEG. Your precious metal shirts are meant to be handwashed, so as to preserve the ink prints and the integrity of the fabric. Woolite is expensive, but it is an investment in your metal cred. In recent years, WOOLITE DARK has been introduced, formulated for dark clothes exclusively. USE IT! Immortal would endorse no less.
2. COLD WASH ONLY. Allow your clothes to soak in waters as cold as the rivers of Blashyrkh itself, without agitation. HAND WASH, or use the “hand washables” cycle on your washing machine. Your precious metal shirts get enough violent agitation when you’re wearing them in the mosh pit.
3. NEVER USE FABRIC SOFTENER ON METAL SHIRTS. “Softener” on a metal shirt? Are you kidding? Does the Snuggle bear really look like he knows the difference between Iron Maiden and Iron Angel? Not only should true metalheads welcome and relish Unsoftness, but fabric softener works by expanding the fibers of the fabric, thereby subjecting your battle armor to undue wear and tear. There’s time for that later.
4. AIR DRY ONLY. Dryers are false metal. The cold north winds will serve you well. But keep them out of the sun’s rays! The sun, as always, is the enemy of darkness.