Bitcoin mixing is basically about privacy, while blockchain promotes privacy to it's highest levels, it also makes every transaction accessible to anybody who cares to look, while your identity is not tied to your transactions, anybody can view every transaction you make and many people do not like this so they use bitcoin mixers to mask their transactions.
And other people use that if they are hiding something and they don't want to reveal where the balance came from. Even gamblers use this mixer so they can convert it to Fiat easily without being questioned where the balance came from before they convert it to fiat because some exchange has strictly rules against gambling.