Is there a reason I should prefix my CSS properties while also using autoprefixers?

Question

I have been working on projects with legacy code for a while now. Even though those projects use autoprefixers for Gulp or Grunt, I still find prefixed properties in the SCSS and LESS files I'm hesitant to clean out like these :

-webkit-transition: width 0.1s ease-out;
-moz-transition: width 0.1s ease-out;
-ms-transition: width 0.1s ease-out;
-o-transition: width 0.1s ease-out;
transition: width 0.1s ease-out;

I asked the question to previous devs already and 95% of the time the answer is "I don't know, it was like that before, better not touch it"

Is there a reason CSS properties should be prefixed while using autoprefixers or are there exceptions I'm not aware of ?