There are a variety of cards that are larger than normal. They're mainly for show since they're not legal or practical in most formats. However a few do have "practical" uses - here are some examples:
- The particular card you reference, Garruk the Slayer, was created for the Magic 2015 prerelease and is used in a variant where one player plays normally and the other plays as Garruk. In that variant the Garruk card starts face up and never leaves the table, so it doesn't hurt that it's oversized.
- In the casual Commander variant, you can use an oversize card as your commander - a commander starts the game face up on the table and tends to stay that way, so as above it doesn't hurt that it's oversized.
"Oversize cards" is not a "set" by the usual Magic definition, but I'd guess your online store categorizes them that way to make it clearer to customers they're buying an abnormal card.