#Because monster quirks bring them to life

There is indeed an important mechanical difference between a Spectator and the banshee you chose as your example: Aberrations are not dead. The Death Tyrant, you will notice, has the Undead type and not the Aberration type. On the other hand, Mind flayers are Aberrations too, and it is their choice of diet that defines them as a staple of D&D. The need for food and drink of the Spectator is not quite as vital as all that, but it's still part of its flavour.

Frankly, the ability to create food and water is not the most 'complex' trait of the Spectator, from a DM's point of view; its madness is, if you want to bring it forth well. First of all you need to determine what form this madness takes (the Madness tables in the Dungeon Master's Guide will certainly help), and then you need to think about how it affects the Spectator's reactions and decision-making.

But little, or big, details like this help make monsters feel unique. If every guardian monster didn't need to eat or drink, they might as well all be golems. The Monster Manual is there to give you inspiration.

You are, of course, always free to modify any monster however you see fit for your campaign. And even if you and your players simply cannot come up with ways to play such a quirk out, you can safely ignore them, nothing bad will happen. But imagine a Spectator refusing gold, but letting itself be bribed by a delicious and moist cake, because decades of eating bland created food make it unable to resist the temptation? That could become a scene to remember.

Because monster quirks bring them to life

There is indeed an important mechanical difference between a Spectator and the banshee you chose as your example: Aberrations are not dead. The Death Tyrant, you will notice, has the Undead type and not the Aberration type. On the other hand, Mind flayers are Aberrations too, and it is their choice of diet that defines them as a staple of D&D. The need for food and drink of the Spectator is not quite as vital as all that, but it's still part of its flavour.

Frankly, the ability to create food and water is not the most 'complex' trait of the Spectator, from a DM's point of view; its madness is, if you want to bring it forth well. First of all you need to determine what form this madness takes (the Madness tables in the Dungeon Master's Guide will certainly help), and then you need to think about how it affects the Spectator's reactions and decision-making.

But little, or big, details like this help make monsters feel unique. If every guardian monster didn't need to eat or drink, they might as well all be golems. The Monster Manual is there to give you inspiration.

You are, of course, always free to modify any monster however you see fit for your campaign. And even if you and your players simply cannot come up with ways to play such a quirk out, you can safely ignore them, nothing bad will happen. But imagine a Spectator refusing gold, but letting itself be bribed by a delicious and moist cake, because decades of eating bland created food make it unable to resist the temptation? That could become a scene to remember.