It ensures that they're protected against a trivial and fairly outdated set of threats. You might say that's better than nothing, but questionable protection leading to a sense of security is more dangerous than having nothing and knowing it (also, false positives).
> We are protected immediately
Look at the recent log4j vulnerabilities. Hosted WAFs didn't "immediately" address the issue (though they were fairly quick). However, to the best of my knowledge they were unable to fully mitigate the threat. For example, if you look at CloudFlare's wording on rule 2c5413e155db4365befe0df160ba67d7:
> In addition to the above rules we have also released a fourth rule that will protect against a much wider range of attacks at the cost of a higher false positive rate. For that reason we have made it available but not set it to BLOCK by default
It's clear that, by default, their updated rules didn't fully mitigate the issue. Also, it IS NOT clear, whether the extra rule did fully mitigate the issue.
Fundamentally, having a WAF in the face of log4j did nothing for you. You still have to patch everything, you still had to try and figure out if you'd been compromised, and you still had to update/alert your clients/customers.