It is very cheap insurance for knowing with virtual certainty that your data exists. Egress is expensive ($100/TB last I checked), but in the target use - backups of last resort - I will be very glad to pay that to have my data back. And if all goes right I'll never pay it.
Personally I find ordinary S3 far too expensive to justify. Not so with Deep Archive.
Note, I work for AWS. Opinions are my own.