The most important advantage of using a shield with a one-handed weapon is that you can only advance in shielding while wearing a shield (or a spellbook). If you only have a sword or another weapon equipped, your shielding skill will not grow up, even if you successfully block hits.
Also note that you need a shield to increase your shielding skill. Using a weapon without a shield won't work, unless you are using a training statue.
https://tibia.fandom.com/wiki/Shielding
Shields are also often not that expensive, offer additional boosts (to the shielding skill, to physical/elemental protection, etc.), and have room for imbuements specific to shields that you cannot apply to weapons.