Nah, the --expand parameter isn't about whether the domain will grow in the future, it's a way to tell the certbot client (other clients might be different, check their documentation) that even though the certificate you're now asking for has extra names, it should be considered a replacement for an existing certificate, not a completely fresh "lineage" of certificates.
Suppose you have one cert for a.example and b.example, and then one day you want a cert to cover a.example, b.example and c.example as well.
If you use --expand when requesting the newer certificate, certbot knows you mean for this new certificate to replace the one used for a.example and b.example today, which is fine since it still has those names in it, just it will now have c.example in it too.
If you do NOT use --expand for that request then certbot assumes you want to keep having certificates issued with just a.example and b.example, but you ALSO want certificates with a.example, b.example and c.example. Every couple of months it will renew both types of certificate.
Some people might want this, but most people will want the --expand behaviour.