Argument: (DMG page 136) Some items require a creature to form a bond with then before their magical properties can be used. This bond is called attunment, and certain items have a prerequisite for it.
Comparing two items in the DMG. Weapon of warning. Weapon (Any), Uncommon (Requires attunment)
This magic weapon warns you of danger. While this weapin is on your person you have advantage on intiative rolls.
Weapon, +1, +2, or +3
Weapon (Any), Uncommon (+1), Rare (+2), or Very rare (+3).
You have a bonus in attack and damage rolls made with this magic weapon. The bonus is determined by the weapons rarity
Note that the weapon of warning states that it requires attunment whilst a weapon +1 does not.
Both of these are DMG page 213
Many items in the DMG do not require attunment to function: alchemy jug, armour +1/+2/+3, Boots of Elvenkind, broom of flying, Cloak of the manta ray, dagger of venom, a deck of many things, dimensional shackles, Dragon slaying sword, Ever smoking bottle, gem of brightness, Gloves of thievery, horseshoe of speed, Iron bands of Bilarro, Mace if smiting, Mariners armor, Necklass of fireballs, Peripat of health, any potion, Quiver of Ehlonna, Ring of swimming, Robe of useful items, Rod of security, Sending stones, Sphere of annihalation, stone of controling earth elementals, Viscous weapons, wand of magic missile, weapons +1-+3, To name a few. Of 62 pages of Magic items in the DMG only 15 of those pages are exclusivly of items that require attunment.