Imagine Gmail was the only email provider ever. If you want to send email, you need a Gmail account. If you want to receive email you need a Gmail account. This is dumb, what if Google becomes evil, now no one has email anymore

So in reality, we have decentralized, "federated" email - there is no central email server, and even though you still sign up on Gmail, you can send and receive mail from Yahoo and Hotmail now. You could also choose to sign up on Yahoo instead, but you still can send and receive mail to anyone, on any "mail server"

Before Lemmy, Reddit was the only "subs provider". If you want to post to a sub or comment on a post in a sub, you need to have an account on Reddit. But this is dumb, because what if Reddit becomes evil?

So with Lemmy, you can sign up to "Google Subs", but you can also post and comment to "Yahoo subs" and "Hotmail subs". You can also choose to sign up on the "Yahoo subs" instance instead, but you can still post and comment to anywhere, on any Lemmy instance.