Money laundering is the processing of criminal proceeds to disguise their illegal origin.
Let's say I were to give you a wad of cash, and asked you to deposit no more than $800 per day into several accounts that I control. For providing this service I give you permission to keep 5% of whatever amount you deposit.
Would that be washing? Maybe. Would that be illegal?
If those funds were legally obtained, then no, that would not be illegal. (From what I know. IANAL)
There are legitimate reasons to mix coins. If my customer that paid me can tell how many bitcoins are in my wallet, I might then be disadvantaged at negotiations if I cry poor mouth yet it is discovered that my wallet is fat, for example. Or perhaps I don't want to become a target by a hacker should my wallet size or identity be determined from the blockchain data. Mixing helps me retain financial privacy.