Ah it's all just different preferences and such. I have a photographer friend who has an iPhone, but also carries a Samsung Note with several camera apps installed, which all have different pros and cons. The newer GCAMmod has various pros/cons vs the Samsung camera app, basically superior low light and still image performance, but less optimal video shooting. It's one of the things that is being worked on currently : stripping the Samsung stock camera app to be an option to add to Galaxy phones running future builds of Lineage.
I'm not a fan of Samsung pointlessly re-inventing half the core apps of Android lol. The last one I had stock was an S7 T-Mobile, and it was jam packed out of the box with ludicrous storage usage.
Probably the primary reason to go custom Rom besides just being someone that enjoys deeper customization and less bloat, is the ability to maintain support longer, which dovetails nicely with the dev community really getting a handle on all the quirks of a particular device and making outstanding Roms for them. The S9 isn't quite in that territory yet, but say you have an S4, 5, 6, or even the still excellent piece of hardware with the 7 or 8. Samsung ended the 7 with Android 8, and ended the 8 with Android 9.
Here's bug-free Lineage Android 10 for the S8
And S7!
Samsung killed S6 support with Android 7, yet Evo-X has a REALLY nice Android TEN version for it :
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If you don't keep phones that long, or don't want to repurpose them for perhaps a nephew/niece/buddy later on, then long-term support and capabilities probably matter little to none. Apple is also varying degrees of awful with long term support, often their 'official' support for older iOS devices just ends up making them slow, clunky, and painful. While running Android 10 on S6 and up is honestly really really nice. And because I like a lean OS with only the specific things I want to use installed, I prefer being able to build a minimalist experience. I also acknowledge that it's not a path for everyone though, at this point I've done many many hundreds of phones and thousands of rom installs, and it's very comfortable and I can tell the difference between a WIP and a daily/primary class of build. I don't recommend someone who is new to the whole thing just jumping without doing a lot of research and having a good reasoning behind the choice to do so.
There's nothing wrong with preferring Touchwiz and the stock experience either of course. YMMV.