Hello, richardzulu!
Welcome to the discussions.
In answer to your question, the clips behind the one you delete must move over to close the gap between the clips, an empty space left by the clip that you deleted. Otherwise, you would have a black/blank part in your movie.
You CAN purposely create a black clip if you wish, but probably you just want the movie to continue as you have created it, leaving out the deleted clip. If your movie is just video, it should work out to be the same once the deleted clip is gone.
However, if you have added some other audio, and not locked it to its corresponding video clip, the timing of the audio to the video will be off. Use the "Advanced" tool to select 'Lock audio clip at playhead' to avoid this. Once the audio is locked to a specific video clip, edits to other clips can be done without separating the audio and video. Occasionally, they still do get misaligned, so you do have to keep checking.
If you really want to keep all the other clips the same without doing anything else, switch to Timeline view, and move the trailing movie over from the clip you want to delete, making sure that the length you move is the same as the clip to be deleted. Then, delete the clip. iMovie will create a black clip in that space and the rest of your clips will remain untouched. However, as I said, the movie will be interupted by this black clip.
