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. The 50 over the 35 because it acceptably renders faces without too much distortion -- the 35 is unacceptable in this regard. In instances where 50mm is too narrow, use the 24 where. Buy older smaller AUTOFOCUS prime lenses so you will actually use them instead of leaving them at home.