Mario, for example, doesn't have that many approach options on the floor, but he has great air mobility and his fireballs are better in the air since the angle they go makes is easier for you to pressure someone and hopefully run for a grab.
All of his aerials but his Forward Air are basically instant, which is great for applying pressure on someone at any time, no matter where and how they are. Not only that, but his aerials all have their own distinct purpose: Neutral Air is a combo breaker and good for approaching, Down air is good for spacing and is a multi-hit, Up air is a kill move when used properly and can start juggle combos at low percents, Forward Air is a really strong spike that can be used to kill very early offstage, or you can use it as a surprise attack if someone approaches you with an aerial, and Back air is also a kill move, it's really strong and crazy quick, and it can space very well.
Every character has different uses for their aerials. That's why they're used so much.