Canon makes approximately 75 lenses, Nikon has around 80. How does someone pick what lens they need? Do I need a prime? Or would a zoom fit my needs better? Why are there so many? What should I make for dinner?
As a new photographer, and at times as a seasoned, all these questions overwhelm us. We see others with flashy new cameras and lenses and we see their photos and think, "if I just had that I could shoot better." Many cameras come with a kits lens that has a decent zoom range, but no fixed aperture. These lens work fine for normal everyday use and random vacation photos. Personally I feel that the biggest upgrade in a lens is a fixed aperture. A lens that stays at f4 for the whole zoom length will be better than a lens that changes from a f3.5 to a f5.6 as you zoom.
As far as the NEED for different lenses. This is a personal CHOICE. I have a friend who is an amazing wedding photographer in Kansas. He shoots 90% of his weddings on a manual Zeiss 50mm lens and most of his images are mind blowing. I have another friend out in Arizona that is also a wedding photographer who produces outstanding images and he prefers to shoot with 4 or 5 different primes. Other photographers us a 24-70mm 2.8 and a 70-200mm 2.8. Each person is different.
But in the end it all comes down to how you use your equipment. If you really want an upgrade Check out Canon's 24-105mm f4. (Nikon's AF-S NIKKOR 24-120mm f/4G) This lens is great for everything from families to kids to seniors. Even an occasional wedding can be shot with this lens. and you can find it used on eBay for $550. Or if you are looking for something even cheaper. Canons 50mm f1.8 or f1.4 (Nikon makes the equivalent lenses) are great cheap options to upgrade.
Don't worry about acquiring more gear. Focus on the content of you images and the rest will come.