Designers & developers can always feel encouraged to create everything from scratch of course, but sometimes using established styles is a great alternative. Your users are probably already familiar with these UI styles from seeing them on other sites they visit (Bootstrap for example is used to build a variety of popular sites). Established code libraries also give you back time to focus on other design/development problems.
You don’t have to be completely original at all times. It’s like cooking: you could go to the store and prepare everything from scratch or you could subscribe to a meal service (like Blue Apron) where you cook the food at home but the elements are given to you ready to go. Use originality as a spice rather than as the whole meal. Using established styles & libraries makes for a good user experience but it also saving you time in the design kitchen.