Wagner Tubas -- Why?

I've been reading about Wagner tubas and I've been wondering how I can determine whether I want to use it in a composition. I know the purpose of the instrument is to bridge a gap between the timbre of conical and cylindrical brass, but listening to demonstrations has left me wondering why I couldn't just use a tenor tuba or regular horn in F in its stead. Some composers such as Wagner (obviously), Respighi and Holst seem to favor it for certain solos, but to me, it doesn't have much of a notably unique sound. In fact I've heard that Wagner tubas and tenor tubas are essentially one and the same, except that one has valves whereas the other has pistons.

Has anybody scored for this instrument, and how do you compare it to various other brass instruments that sound similar to it? In other words, why favor this one over another, or have it as an extra in the orchestra?

First, Wagner tubas are designed to be played by horn players, trombone or tuba players are just apart, they won't mess with this, on the contrary, euphonium or baritone horn are played by trombone players (maybe tuba player) so in your orchestration you decide which instruments you will lose when switching to wagner tubas.

Indeed, sound is very similar to saxhorns, but I'm sure (without being a brass player) it does have an utility, since both were created by Adolph Sax and he could have just told to Wagner "hey ok why don't you use my Saxhorns for that ?" instead he built the wagner tuba as wagner requested.

I'm just guessing but thinking in wagner tuba as a horn, and baritone horn as trombone, wagner tuba should be more appropriate for pp passages maybe ppp<ff dynamics without changing so much in timbre, (warm to bright).

If you don't really know how to use wagner tubas and you don't really need them on your score, don't use them, I haven't.

Alright, that does sort of clear it up a bit for me, thanks.