The properties for video strips are an excellent resource for video production in Blender, but they will become much more useful with animation. Blender has a full animation system, and you can animate almost all properties.

How do you create animation? Blender is interpolation-based and will work with keyframes to make animation. In interpolation, you will need at least two keyframes in different locations in your timeline. If those keyframes have properties with unique values, Blender will calculate all intermediate frames to make a transition.

For instance, you can start in frame 1 with a strip having an opacity of 0. Add another keyframe for that strip in frame 48 and set the opacity to 1.

That will mean you have a strip starting with full transparency and, in 48 frames, which is about two seconds, it will become fully visible. The transformation won't be instantaneous, but gradual. Blender will do the math and create all opacity changes between frames 2 and 47.