> Isn't concurrency also limited by your machines disk speed for writes, what difference does it make if you write sequentially vs concurrently? Why does concurrency even matter for databases?

For a simplified example, having three processes reading blocks X, Y, Z in parallel is much faster than having a single process read block X, wait for the read to finish, read block Y, wait for the read to finish, read block Z and wait for the read to finish.