Being able to trim video clips allows you to remove the ‘rubbish’ bits! These can include the starts of clips when you’re waiting for something to happen or the ends of clips when you forgot to press stop!
Traditionally, adding a crossfade between video clips is used to indicate the passing of time (E.g. Moving from a day scene to a night scene). You’ll also find a crossfade refered to as a ‘cross dissolve’, as is the case in Premiere Pro. This transition is appropriate for use in the spring flowers video as long fades fit well with the relaxed pacing.
Fading in from black is often used at the very start of a film or section/scene, while fading out to black indicates to the viewer that scene is over.