I'm copying what I typed from another thread that was similar. :)
It's actually incredibly beneficial for balance purposes and for publicity's sake to release champions that are stronger than they should be. Releasing a champion that is too strong isn't a very bad thing, but releasing a champion that is too weak can be an incredibly bad thing. The bad that comes from releasing an underpowered champion is greater than the bad of a champion that is perceived as strong on release, and here's why.
When a champion is first released, nobody knows how to play him, and nobody knows how to play against him. This means that there is hardly any useful data for the balance team; without stable, consistent data, making balance changes is tricky. In order to know what to balance, people need to learn how to play the champion. What champions do people try to learn to play? The ones that are powerful. After people learn how to play these champions, they can use the statistics and truly say, "Okay, now that we have data from people who are actually learning this champion, we can tune him back a bit and make him balanced."
In short, Rito gets balance data faster if they're OP, because people will learn the champion, and the balance team will be able to know not just that the champion needs a nerf, but WHERE to nerf the champion. In the case of Ekko, is his shield really the problem, or is it his ultimate damage? Or could it even be his passive? Right now, the numbers are all over the place because he was just released. After a while, they'll know what's consistently strong and be able to fix it.
Releasing a champion that's underpowered on release is a nightmare for balance purposes. People play the champion, realise he's weak, and then stop playing the champion altogether. This is very very bad for balance purposes, because now the needed data is non-existent. The balance team can kind of guess what the problem is in an underpowered champion and try to buff it, but it may not even be the right thing to buff. Furthermore, it often leads to overcompensation - an underpowered champion gets buffed to the point where he's now OVERpowered.
Of course, it won't SEEM overpowered, because since people refused to learn the champ on release, now nobody knows the potential of the champion, and the data once again becomes scattered as players begin learning the champion for what we can consider the first time. Now we're essentially back to square one, and the balance team has to wait even longer so they can get more useful data to know what to change to balance the champion out.
Did you read Harry Potter? What made you want to read the second book? Obviously, the first book was good. What if people told you the second book wasn't quite as good as the first book? Would you still read the second? Of course you would, because the first was so good, you'll at least give the second a chance. You'll know it's not quite there, but you're already into the series now.
Imagine if Harry Potter's first book was incredibly dull and boring. You hated it. Then somebody came up and told you that the second book was better than the first. Would you believe them? Would you really want to try that second volume after thinking the first book was terrible? Even if the second book was better, you'd hesitate to try to read it until it appeared on the NY Time's Best Seller list.
Champions work in similar ways. If a champion is really weak on release, you'd get a bad taste in your mouth for that champion. Even if a buff came around for that champion, would you look at yourself thinking, "Boy, I'm super excited to try him now!" Probably not. Not until LCS picked him up, at least. He'd always seem weak in your eyes until somebody else showed you the buffs were okay.
However, if you nerf a champion that was strong on release, people will have already learned the champion and will be sitting, reading the patch notes, thinking, "Okay, so Ekko's ult ratio is now 0.9 instead of 1.3. That's not so bad. He'll be fine. I still enjoy the champion and I'll still play him."
And that's precisely why I do not mind having a champion on the strong side on release. While frustrating sometimes, and maybe not as "exciting" or having the underdog mindset, it ultimately is a rather smart thing to do from a big picture perspective.