many reels are multi-purpose reels. a good 6.X:1 reel can be used for just about everything. 7.X:1 reels are needed for techniques where you need speed. If you hook a bass in heavy weeds, or heavy cover, you want a blazing fast reel to get it up and out of there before he wraps you up and breaks you off. Like flipping, pitching, frogging, Carolina rigs etc….., any technique where you need to get the fish away from trouble quickly or need to move the bait quickly is good for the really fast reels. 6.X:1 reels are good for spinnerbaits, chatterbaits, jerk baits, topwater poppers and spooks, twitch baits, squarebill cranks, lipless cranks, etc…. 5.X:1 are for deep cranks with the big lips, heavy slow rolled spinnerbaits, big swimbait, Alabama rigs, anything that is heavy or creates a lot of drag you need the power and not so much the speed.
This is just an idea, nothing is etched in stone, every fisherman has his favorite techniques and will use the setups he likes best for those techniques, you can use a 7.X:1 for deep cranks if you want but you will defeat the purpose by getting it out of the strike zone to quick, and you will be wore out in a couple of hours from reeling a deep crank with that reel, it just doesn’t have the power of a 5.X:1 reel and a deep crank will feel like you have a brick tied on after a few casts.
I would suggest to start, get you quality baitcast of each reel, 5.X:1, 6.X:1, 7.X:1, and get 2 good spinning setups and you should be good for just about any techniques. once you get a little experience, you will know more what fits your style and the equipment you need to fish it effectively. hope this helps a little.
Read, read, read, watch videos on you tube and BPS free videos on their website, fishing mags. the pros will tell you the equipment they are using and why. use your computer like a fishing guide, ask it to show you what you need and how to fish it and why. there are so many good online resources and I use them all constantly. its gratifying to read about something, then try it and see it work where you did the research, and the work all on your own.