I think for this app, the purpose is a convenient and secure writing space that's available across devices. I've been a Standard Notes customer for several years now, and I went with it because I was looking for a secure solution for my journaling. I wanted a secure place to write my deepest thoughts at home and on the go, and also a way to quickly and easily access, edit, and review those thoughts across all of my devices. That's a tall order.
This conflict of desires lead me to the age old trade-off of security versus convenience. If I want security / privacy, I should use only local text editing tools and keep everything on an encrypted volume, preferably on an airgapped machine. If I want pure convenience, I should use any of the million great cloud-based note and document options that are available on any device. If I want to fall anywhere between those extremes on the spectrum, I'll need to decide the most convenient option I can bear that provides the most security. And for me, I landed on Standard Notes when exploring this question for myself.
It's a lightweight and easy text editing program that is encrypted and secure anytime my writing leaves my sphere of control. It allows for search of my notes, filtering by title and date created/modified, and it has great sync between all of my devices to the point that I can pick it up any device and write a thought as it comes and not worry about sync conflicts if multiple instances of my note are open across devices. There are easy options to automate encrypted local backups and even daily cloud-based ones if you're a paying customer. I can export everything to plain text quite easily - which is my preferred format - so I'm in no way dependent on their continuing to exist.
If you want to read/write notes seamlessly across all of your devices and you also want to stay private and secure, your options grow limited right away. Standard Notes is the best compromise of these competing interests that I've found so far for my own situation. So even though I have the ability to use other tools as you've described - and many of them are better writing tools than Standard Notes - I still choose to pay SN as a customer.