Design is about contrasts and defined movements there is no middle ground and nothing is left to chance. Beautiful things aren't semi-glossy they are a combination of gloss and matte. Likewise when interacting with a product interactions should be decided. Does it fade? Then it fades fast or super slow. Does it slide then it slides fast or super slow. Medium to slow feels like it's heavy, badly coded and undecided delaying the user from achieving their goals. Fast shows that it is tight and light. Super slow is there for effect it’s used for non-essential background effects and to give a page depth. Never ever keep the user waiting longer than they need to.
Design is relies upon metaphors pulling references from the world around us. We try to imitate nature. Things don't appear out of thin air. They grow, expand & move. We in web have a page load which is a jarring experience, a little like Alice in Wonderland. You climb into the rabbit hole and you come out somewhere else. You may end up where you expected, you may come to a 404, you simply don't know. Transitions/animations make that transition of where the fuck am I a little easier and how to get back a whole lot more safer.
While there is a lot more to user experience than this. All you need to know is that I know this and I will do my utmost to alleviate friction for your users. Whether it be in the code that I write or the wireframes I create.