Fat bikes are purpose-built for soft, loose surfaces where normal mountain bike tires would sink and struggle. If you regularly ride on snow, deep sand, or ultra-loose terrain, a fat bike's massive 3.7-inch-plus tires provide flotation and traction that nothing else can match.
For most riders on typical dirt trails, a fat bike is overkill. They're noticeably heavier and slower on hardpack and rolling terrain compared to trail or XC bikes. The wide tires create more rolling resistance, so you'll work harder on regular trails and paved connectors.
Consider a fat bike if you live somewhere with long winters and want to ride year-round on snow, if you have access to sandy desert terrain, or if you're planning an expedition-style tour over unpredictable surfaces. Some riders do enjoy fat bikes as a fun alternative on regular trails—the traction and stability feel unique—but if your local trails are mostly packed dirt, you'll likely be happier with a trail bike.