[SampleMan]

Of course biological features each have some function, almost without exception, so what exactly are you talking about?

Not really. What they do always have is a presumed function. Tigers need stripes and that serves a purpose, but lions and jaguars do not need stripes. A rather self-fulfilling premise. Do the stripes actually serve a purpose? Ditto for stripes on a zebra, do they serve a purpose? If the zebra had the same coloring as a gazelle you would never ask, why doesn't it have stripes. Obviously a fishes fins serve a purpose, but other features are just a guess. And if a tiger's stripes do provide a specific advantage, did the stripes evolve because they provided the tiger an advantage in its current environment, or did the tiger gravitate to environments where the stripes it was cursed with were actually an advantage?

Impossible to know in many cases.

[BroJoeK]

SampleMan: "What they do always have is a presumed function.
Tigers need stripes and that serves a purpose, but lions and jaguars do not need stripes."

So I take it, you are not asking for the philosophical distinction between "function" and "purpose", but are treating those words interchangeably.
That's fine.
I think I'll stick with the more practical word, "function".

You ask about lions and tigers, and the presumed function, or lack of it, of stripes.
So, first we notice that cats living in deep jungle are all spotted or striped, and that makes them more difficult to see when hidden amongst leaves & trees.
By contrast, cats living in more open & dry ranges are more plain colored.

If you ask why & how, then Occam's razor suggests the simplest natural answer: they evolved to take advantage of their environment.
If you ask, why didn't every species evolve the same features, the answer is: in many respects they did, for example, can you think of any cats with other than four legs?
Clearly some features apply universally.
As for other features like size & coloring, those must surely be adaptations for very specific environments.

Bottom line: science can only go so far, can only say so much given available evidence, but the basic idea that creatures adapt to their environments is well established, indeed is not controversial.
So why make a big issue of it?

SampleMan: "...but other features are just a guess."

Features which are not helpful quickly devolve in nature.
So, if we see no obvious reason for some characteristic, then the next most likely explanation is: because females like it (i.e., birds of paradise), or sexual competition (horns on rams).

Consider the devolution of whale legs: