Many Lemmy instances have their own focus, themes, vibe, knowledge, culture, and more. Searching exclusively within local is useful for that. If you want broad federation, then Lemmy.world is ironically worse than instances like Lemmy.ml, because Lemmy.world blocks leftist instances like Hexbear.net and Lemmygrad.ml. Lemmy.world is basically Reddit but FOSS, with all the good and bad that comes with that, while other instances are genuinely unique and interesting by themselves. Federation is just a bonus that adds variety and more engagement.