For newer players, I'd understand that you need to own at the least 20 champions in order to start playing Ranked but from what I've seen, most people obtain their champions from rewards and Hextech chests. Blue essence is kind of useless once you have all the champions, I wish there were more ways to utilize it rather than just buying Urfwick for 150,000 BE or purchasing chromas. There's literally no point in buying champions with RP unless you were just one-tricking that champion on a smurf account or you want to quickly buy the newest champion out to test them (although some do that for the sake of playtesting, you can just easily wait for Riot to include them in the free champion rotation).
This goes the same for ward skins, summoner icons, boosts, rune pages, etc.
I'm surprised Riot is still discounting champions for RP, if you didn't know about this just go to Shop > Champions > On Sale, check the "Show Owned" box as well.