This way you can at least force her to max W
If Sona is doing that against Leona, she's making a huge mistake. W doesn't give enough shielding and regen to do anything, and it guarantees that you can't do enough damage to Leona's ADC to dissuade them from going in again. It also takes away from your ability to poke the Leona and her ADC out level 3-5, which is a huge part of the matchup.
I don't care how aggressive the Leona is, I'm maxing Q. There is no situation where I wouldn't. Even we're losing hard, I wouldn't max W, because that just guarantees that we'll never be able to get a cross kill in a gank, and it means that I won't be able to do enough damage to roam effectively at 6 (and roaming at 6 or even before 6 is exactly what I'll do in a losing lane).
I think the problem is people pick stuff like Leona or Thresh into Sona thinking it's gonna be ezpz but really, you still have to play the lane way more aggressively than you might normally play it to really get an advantage over Sona.
I kind of disagree. I actually think the problem is exactly the opposite: People pick Leona into Sona thinking they can win every trade, and as a result they go in literally all the time, and they play up on you thinking that the threat of an engage will make you back off. The thing is, Sona's heal and poke design is based around taking trades that are even slightly bad, and punishing them horribly, and she can actually resist getting bursted down thanks to her shield/heal. Pre-6, if Leona and her ADC don't get all of their burst off very quickly, Sona can turn it into a sustained trade that she and her ADC will almost always win. After 6, Leona's ults have to be followed up very quickly, and she has to go in while Sona isn't in a position to stun her ADC, or else Leona will lose most of her stun uptime and sunlight procs while her ADC is getting bursted. The impetus is on Leona to take only the best fights, and way too many people will just go 100% ham 100% of the time.
Ever since Sona got the shield added and had her base stats increased, I actually think that Leona really doesn't do great against Sona. If the matchup is played absolutely perfectly, Leona will win, but even small mistakes open her up for a ton of punishment. I will specifically pick Sona into Leona because of this.