I've seen some people put points in various apostolia color lines. Why? I dont see how is it worth it or is it just that they literally have nowhere else to put them?
You're correct - they may not have a better option to put their stats.
At 410 you're able to max out 3 stats, with 266 extra out. I'm talking 461, 461, 461 of your 3 primary stats. And this will be higher if you're overcapped.
If you're a mage, after maxing int dex vit, what's next is to either add blue/green lines for more matk, or to explore some evasion build.
If you're an atk class, you may want to add in green/red lines to further enhance your att.
If you're a full support using disable, adding to get more auto% is one viable way too.
My recommendation for the top-fastkill-solo class will still ultimately be Alchemists due to Damage disable sets and defense-ignoring Ailment damages. It's nigh-impossible for any other class to easily catch up, even with the introduction of Lv360 skills in the far-off future.