Very often when you are doing something creative the end result is not known. It is an experiment, an investigation.
You use the images and ideas in your head to lead you and fire your engines - along the way you often make many, many new discoveries.
Discoveries that in themselves can demand whole new investigations.
If you do start that next investigation you trade the potential of your current idea for the opportunity of this new one - it was unavailable and invisible to you before, but not anymore.
This is a great dilemma for experimental processes - how do yo know you have reached a dead end, how do you know when to 'give up', how do you know the value of the alternatives over your current objective.
Time is finite. So choices about how we spend it come with real costs.
It is easy to sway in favor of the opportunity. Early inspiration, too little knowledge, full of naivety for the new and burdened by experience of the old can lead to us constantly ditching our projects in the middle - just when it is most important to push through.
Finishing is important, but so is knowing when to quit.
But more important is remembering why you started in the first place.