I looked the term up and still have no idea what fishtail trousers are other than from the pictures that all show people wearing pants that don't fit.
What are fishtail trousers and what advantages do they offer?
They're high-waisted, fairly roomy if you like, or if your figure allows, as tight as you want in the upper regions. The fly is usually button, and they generally have buttons for braces/galluses/US "suspenders" (in UK an item of women's underwear or men's sock hold-ups) and sometimes a cinch-back strap (like old Levis). If you prefer to hold your trousers up with braces from your shoulders rather than a belt, then these will do.
The fishtail comes from the cut of the back of the waistband and the v-shape. I'm no tailor, but I think this styling is partly decorative, partly traditional (like watch pockets on jeans, ticket pockets on coats etc.) and practical, to allow the high waist to fit the body correctly.
This style is not for those who like hipster trousers, or to have several inches of their underpants showing above the trouser waistband. Unless they're made to fit, they're perhaps not the best trouser for those gents with a fuller figure and waistline. As with other vintage styles such as USAAF flight jackets and so on, they were in fashion when men were less well-built than today. I'd go so far as to say that they are best if bespoke anyway, and my own view is that they can be ill-fitting if not so made.
As to the advantages, the features I mention may or may not appeal, or offer any benefits, and, as with most clothing, it's down to preference, personal style, comfort and the ability (or wish) to wear things to best advantage. Again, as with any outfits, they may not be the most appropriate style for some people, yet they will wear them.
If YMMV is important in shaving requisites, that palls into insignificance when it comes down to the serious matter of trousers. And they thought Wallace had problems....!
The Wrong Trousers