It's simple in that if you are already using an s3 compatible object store you don't have to worry about setting up a separate redundant, durable, highly available, and possibly scalable storage system for your logs. And as for the implementation of this or Loki, it means they can delegate most of the complexity of data storage to a separate object store.
well, what if I just want to run this on my laptop...?
It looks like Loki at least supports using the local filesystem as an object store. I don't know about Parseable.io.
But also, running on your laptop isn't really the intended use case.
run minio locally
But the "S" in "S3" stands for "Simple".
What does the "3" stand for?
It means there are 3 S's. "Simple Storage Service", or "S3". (See https://aws.amazon.com/s3/ — "Amazon Simple Storage Service (Amazon S3) is […]")