No matter how casual your outfit is, even with slippers.
The moment you add a blazer, everything is just better. It gives it a touch of sophistication, it stylize your trunk, and it grants you authority.
If your outfit is very monotonous, do not hesitate to give it some color. If you have printed jeans, be sure to give a serious touch to the blazer. Rocker, romantic, bohemian or glam, the blazer suits all styles and you have millions of models to pick.
Choose according to your height, the style you want to achieve or the formality of the event for which you will use it.