Focal lenghts are simply tools. Different tools for different jobs... For example:
1. I want to shoot an interior of a room. I want to get the entire room in the picture. I better have a wide angle zoom. A zoom to aid in composition. I can't use a tele. I can't use a 50.
2. I want a versatile lens that's great in available light because I don't want to (or can't) use a flash. I want one lens to rule them all. A fast 50.
3. I want to shoot portraits. I want some compression so faces are not distorted. I want to blow out the background. I want a wide ap for available light and/or aid in bokeh. Short tele-length fast prime.
4. I want to shoot wildlife, birds, sports. Telephoto zoom.
5. I want to shoot small items -- products, coins, stamps, insects. Macro (or "micro") lens.
Versatile pairs -- 50 and a 24. 85 and a 35.
Wrenches come in sets. Different sizes -- same general function but vastly different uses. You don't use a the same wrench to fix your bike as you do for your plumbing. Doubtful you can make due with one wrench if you're a mechanic or a plumber. No. You need a set. Same concept with lenses if you're a photographer. Fast primes for your mains. Zooms in the wide and tele extermes. Keep slow mid-range zooms. Useless. (Exception being a small digicam. But those are obsolete for the most part. For the price, get a used full frame camera...)
Yes -- lenses are expensive. Start with a fast 50/1.4. Not a 1.8, not a 2.0. A 1.4. This is the only 1.4 you need. And don't listen to anyone who claims that a 1.8 is "better". No. Buy a good quality used one, let some other sucker eat the depreciation. Next comes a 24/2.8 with which to pair it.