Different colored flies, show up better on different colored backgrounds, some people put a profile plate behind their vise to make more contrast between the fly and the background, this makes it easier to see the fly and know that you have everything in the right place.
if you look at pictures of flies on the Internet you will notice that the edges of some are very crisp and others are hard to define and look washed out. Some of this is profile plate, some due to the skill of the photographer.
Only you can know if its worth it to you to use one.
Eric