Remove or balance Unwanted Tones in your Blonde Hair.
There's so many products out there with blue or purple toning properties. All are great but they do differ a lot in the actual 'tone'.
Blue tones counteract unwanted orange (aka 'brassy') reflects in your hair.
While purple based products work against unwanted yellow tones.
That's hard to visualise if you're not a hairdresser. So... look at the Colour Wheel and simply know that opposites cancel each other out.
To help you, here's a full explanation from one of expert stylist on which product has what tone in it.