Litotes can be used to politely criticize someone without being too harsh or direct. She isn't the fastest worker in the office, meaning she works slowly.
Pro Tip 2/3
Irony Amplifier
Sometimes, litotes can create an ironic effect, implying the opposite is very true. Winning the lottery would not be a small convenience.
Pro Tip 3/3
Subtle Praise
Litotes can be used to give a compliment modestly by downplaying the intensity of praise. He's not a bad singer, actually implying he is quite good.