If you take replication of specific theatrical creations out of the equation, what is the point of building a prop raygun?
It is straight-forward to carve, cast, assemble, print, or whatever a nice looking raygun. And it is fun to do. But at the end of it, you have a gun-shaped object. And you could spend ten bucks at the local toy store for a NERF or water gun. Or spend a few more bucks to get a nice looking thing -- assuming you either don't care or even, rather like that it is a duplicate of something used by the third Stormtrooper to the left on the second film of the first trilogy.
To my mind, there are a few possible reasons to do your own.
First is that it is fun. You are doing it because you want to do it.
Second is to save money. And this is a "Sort of." You probably can't get something as nice as a quality replica prop for less than it would cost to just order one. But you can get something better than a water pistol for less than you'd spend for a quality replica.
And last is to get something that just isn't out there. Leaving aside, again, the question of wanting a replica of a specific (licensed or unlicensed, historical or fictional, whatever) item, this question narrows into uncommon design elements.
The raygun I made for my sister qualifies; it is a type of retro design that is uncommon and hard to find pre-made (well, hard to find pre-made in semi-realistic form).
A direction I've been tempted in for years is towards the sort of materials and fine detailing of watches and expensive lighters. For that matter, there are few fictional hand-arms (available as reproduction or not) that have the sleek hard simplicity and the look of precision machining of real hand guns. The most typical design form is oversized, rounded, sort of lumpy (pretty much any first-person shooter game with a future setting).
So maybe, there is a value in making prop pistols with high-end fabrication techniques; something showing real metals, real woods, moving parts.
All things, in short, not readily achievable in sculpting, carving, casting, 3d printing...in clay and MDF and craft foam and found objects...in short in any of the materials and methods that are fun and easy to do.
I could probably whip out a nifty-looking laser gun prop in a week using the kinds of traditional materials and methods I have lying around the apartment. But I can't think of any good reason to do so.