"Because it Looks Cool" is a shitty reason to put anything in a movie.
Except, of course, that movies are primarily a visual medium and scifi movies in particular have always presented striking, eye-catching visuals.
"Shall we have a giant gorilla climb to the top of the Empire State Building and swat biplanes from the air? It will look cool!"
"Don't be silly. Cool visuals have nothing to do with cinema! We want only sensible, prosaic images that won't run the risk of exciting the audience."
This is not to say that scifi movies are only about the visuals; the characters and story matter, too. But, all other things being equal, why not go for the eye-popping visual over a boring one?
Lord knows TOS knew the value of snazzy visuals. Remember the Tholian Web, or the psychedelic Medusan, or the cool way Losira manifested in "That Which Survives"? As far as the plot was concerned, did Losira need
to turn into a vertical TV bar? Of course not. But it looked neat and was more visually interesting than just having her appear in a flash of energy.
Heck, did Orion slave girls need to be green? Not really. But green dancing girls look cooler than plain old human ones . . . .