Is it the culmination or just a premise?
That moment when the music changes, the flower petals start flowing out of nowhere (you can change them to snowflakes if it's winter or to some maple leaves in autumn), the wind ruffles the characters' hair, and the characters themselves move closer to each other in a slow-mo, with eyelashes fluttering over their sparkling eyes and gasps leaving their tiny mouths in little puffs... Ah, the tension is palpable, they're finally gonna kiss!
Sounds familiar?
Romance anime puts a lot of emphasis on kissing. One of the reasons is that kissing cements the actual start of a relationship, and shows that it's serious, not just a fling. The other is that the majority of the series is still oriented toward the teenage audience, and kissing is still considered the maximum amount of appropriate intimacy for people of that age to show on screen. So having the characters who were pining for each other for one, or maybe more than one season, kiss in the finale is the perfect cathartic moment for the whole story. Sort of a "happily ever after" for the anime romance setting.
But what's gonna happen if the characters kiss not in the finale, but earlier?
The real life begins. We take a peek at that "happily ever after" which turns out not to be so happy. The seriousness has been proclaimed, so now the characters need to apply that seriousness and deal with the specifics of navigating life with another person by their side. This is a hard task.
And this is a different set of problems than the one that usually precedes the kissing. Pre-kissing life most of the time is filled with insecurity, fawning over, adoration, and self-deprecation. Post-kissing, however, starts focusing on compromises, on fitting together, on goals, on the future, both together and apart. Because it already has something that sealed our characters together, tied him in an almost legal way — a kiss.
The plot lines that authors explore after kissing seem much more diverse than the ones that are featured before kissing. In a way, a kiss serves as a border between childishness and maturity, when you realize that now you should take into consideration not only your own feelings, ambitions and goals, but also those of the person tied to you. You can easily drop them before the kiss, but not after.
The kiss in the anime somehow takes on the role of marriage in terms of the rights and duties it invokes on the characters. So its placement in the story affects the characters more than you may think.
Just watch Tonari no Kaibutsu-kun to understand that. Or, even better, read it.