> Isn't concurrency also limited by your machines disk speed for writes

Yes, in theory: given a large enough database, and a disk that can only do one operation at a time, and a large enough operation that touches enough of the database. In practice, in a SQLite single tenant scenario? No, not at all.

> what difference does it make if you write sequentially vs concurrently. Why does concurrency even matter for databases?

As soon as your codebase involves reacting to events independently of a user taking action it becomes a practical concern. Generally, this is a broad question and has 1,000,000 answers.

EDIT: Originally I had "I think you understand generally, no?" appended but realized that's not helpful at all, if you did, you wouldn't be asking.

Something that may help is imagining what'd happen if a DB wasn't thread safe / didn't allow multiple writers. Ex. in SQLite's case, it allows multiple write operations to take place but there's a one-at-a-time queue. If we didn't have databases that were able to execute multiple writes simultaneously, you'd need a separate database for each concurrent writer you expect, and you'd effectively have a global lock. Orderly scaling would be ~impossible unless you did something crazy like have a single server per user