I can give you two answers - the "subjectivist answer" and the "objectivist answer" - which, in this case, sound quite similar.
The subjectivist answer would be "so you have more choices to pick between" (no further explanation necessary).
Assuming that your goal is accuracy, and you're starting with a 16/44k digital audio source, you really need to use oversampling (because doing the conversion without oversampling would require either filters that are impractical to design and build, or filters that cause an obvious alteration in frequency response, or both). Once you choose to use oversampling, part of the process requires that you use filters that alter the signal - at least to a small degree. Our little Ego DACs offer three filters; they all measure as being very flat (steady state), but one has symmetrical ringing, another has less pre-ringing and more post ringing, while the third has less ringing altogether, but is less accurate in other ways. Many other DACs offer similar options. The point is that all three are very close to "perfect", but none of them is absolutely perfect, and many people can hear a slight difference between them. And, since all three are technically compromises, but in different ways, you can't simply declare that one or the other is "the most accurate one". And, if you look at the oversampling filters in other products, especially software conversion programs, many offer various options - sharper or gentler slope, flatter frequency response at the cost of less accurate phase characteristics, and different transient characteristics. And remember that these are all at least somewhat of a compromise because nobody has yet designed a filter that can simultaneously deliver perfect frequency and perfect transient response (ignoring whether the differences are audible or not - they are easily measurable).
Therefore, acknowledging that "the perfect one" isn't an option, the objectivist answer is to either pick one for the user, or let them pick the one they prefer after explaining the strengths and weaknesses of each.
One common solution, and the one chosen by HQPlayer, is to combine both..... They offer two or three choices which sound very similar, and are very close to perfect, and then throw in a selection of other options that are less accurate, and where the differences are clearly audible, and let the user decide for themselves.
One thing you have to understand is that the real question isn't "Why would
YOU want it?"; the real question is "Why would
WE, or any other manufacturer, design a DAC or player program that way?" And, when you look at it from that perspective, the answer becomes more obvious. By offering a DAC that offers multiple options, I have a product that will appeal to both customers who want the most accurate DAC
AND customers who either prefer one that's less accurate - because they "like the way it sounds" - or who simply like lots of choices. If you read other forums, you'll find people who seem to get great joy in "discovering what switch settings work the best with this or that album or song". (It's not much different than the reason why many car dealerships carry multiple brands of cars - or why two or three major tobacco companies offer literally dozens of cigarette "brands" to choose from.)
And, in the case of HQPlayer.... there are lots of player programs out there, and offering a wide variety of choices is a special feature that makes that product different - and so more appealing to some customers. Assuming that you actually are curious about whether the different filter choices will sound different, and which one you will prefer, they're offering you the option of trying out different ones by changing software settings - which is much easier than doing so by building different DACs, and configuring them with different filters in firmware, and much cheaper than buying many different DACs. And, if you're the sort of customer who is going to pick which filter they prefer for playing each song, then you can do that too.
Also bear in mind that, whether we're talking about a DAC or a player program, once the basic design is done, adding additional filter choices is relatively simple - so it's a feature that may appeal to many customers, and one which can be added without much effort or cost to the manufacturer.
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I'm sorry, why would I want a filter that causes an audible difference?