### Because that's how it was in previous editions of the game.

In the D&D third edition book Magic of Faerûn (2001), the Spectator has the ability to _create food and water_ with one of its eyestalks. Its other three were _inflict serious wounds_, _hold monster_, and _suggestion_.

An updated version of the creature in Lords of Madness (2005) changes the creature so it creates food and drink as an innate ability, and one of its eye stalks instead causes fatigue.

It needs to eat and drink because according to its lore, the spectator is dedicated to to guard a specific place or thing without having to leave to hunt for food. Beholders are traditionally a type of creature known as an Aberration, and therefore it would not be correct to simply make them Undead.

Giving creatures the ability to cast a single spell is a long-standing D&D mechanical tradition that dates back to the earliest editions of the game, when the easiest way for designers to give a monster a magical capability was simply to allow it to cast a spell normally available to a player character spellcasting class such as the cleric. D&D third edition often did this, where they were generally known as spell-like abilities.