Prefixes are used to implement some items that are not compatible with other browsers, but exist within a specific browser.
But a lot of CSS elements do not need prefixes and some that they need no longer need.
Ex. border-radius
it is very stable in all browsers other than backdrop-filter
that only exists in Safari webkit for iOS 9 and needs a -webkit-backdrop-filter
How to find out if you need to use the prefix or not?
You can check the caniuse it is very easy to use, just put the element you want to use and see which browsers are compatible. It has an icon indicating when the property needs the prefix.
Although this approach is more labor intensive than using a generator, if you are learning it is the best way to get to know the evolution of CSS 3.