Now, as to "why do we need magic items attached to sneak attack"- this ties into the greater question of "why do we need magic items in the first place"- and for the vast majority of items, we really don't need them. Most magic items are like spice or frosting. They can make the resulting meal more visually pleasing or better tasting. Or if used wrongly, can have the opposite effect.
The only magic items I feel are necessary are ones that shore up weaknesses a given class may have. Things like access to being able to deal with unseen foes, harm enemies resistant to your attacks, improved mobility options, making your more survivable, better able to handle higher level challenges, and so on. Your group might not need a particular item, based on it's composition and how well the characters complement one another- but they exist in case that need exists.
Mostly, magic items are fun.
Following that preamble, "why should a magic item enchance or replicate a class ability?". Here are some reasons, you can take or leave them. Honestly, I don't really understand why this is a conversation- D&D has, over the decades, had magic items that can do any sort of fantastic thing, simply because someone thought it would be fun to have a magic hat that lets you change your class (Hat of Difference), armor that works in wild shape (Wild Armor, Wildling Claps), or even replicate the powers of other classes (Cloaks and Boots of Elvenkind granting up to 99% Move Silently and Hide in Shadows when these were exclusively Thief and Ranger abilities).
But if you need reasons why you might want this sort of magic item to exist, 'ere we go.
1- a class is absent from a game. If you have no Barbarians, a magic item that lets you Rage once per day allows you to experience another class fantasy, and might shore up a weak point of your character.
Example: Gloves of Missile Snaring basically let you mimic the Monk's Deflect Missiles ability. Also Wands and Staves that can allow you to cast spells you donthe shore up weakness but 't have access to.
2- it offers extra support for a class. The previous example of an extra use of Rage can be very welcome by a Barbarian character. Handwraps or Rings that allow Monks or Druids to replicate the benefits of magic weapons for their unarmed strikes/natural weapon attacks, a Belt that increases the effect of Second Wind, an item that improves the abilities of your Familiar or Animal Companion, or any other thing you can think of- if the item interacts with your class abilities in some way, then it enhances your class fantasy.
Example: The Rod of the Pact-Keeper makes you better at using your Warlock magic.
3- World-building and immersion. If magic items can be created, why wouldn't you create magic items that can enhance class or even race abilities (one could see The One Ring as enhancing the natural stealthiness of Hobbits), or even grant them to allies, who lack these abilities! The reasons why and how such things are created can lead to more engaging stories and a more immersive world.
Example: The Girdle of Dwarvenkind, which gives you some of the abilities of a Dwarf.
4- as a limitation. A sword that does 2d6 fire damage is a clear upgrade to anything that isn't immune to fire. It's 2d6 every time it hits, every combat. But a sword that inflicts 2d6 Sneak Attack damage only triggers if you have advantage or you have an ally within 5' of the target. That runs the risk of not coming up every single turn, and forces you to adjust your tactics if you want to use it. This makes it potentially less powerful and more interesting.
Example: well none in 5e that I'm aware of. Certainly there are items that have limitations, like a Dragonslayer weapon only does it's bonus damage to Dragons, and a Vicious weapon only works if you score a natural 20. This is just another potential way you could achieve such an effect.