These 2 statements contradict each other.
The tyres are there for GRIP. Wider tyres at the rear will, in general, increase the rear grip levels, and that may induce understeer. And, if you are going to fit 2 different size tyres front & rear, then you really should fit 2 different size rims front and rear, because tyres have an optimum rim width which they are designed to fit.
My 2c worth.