They enable searching of data they keep unprotected (the metadata so to speak)
And proton mail bridge seems to be an "offline" (i.e. offline to them) email search engine that you run yourself, so it downloads your emails, and makes them fully searchable, but to do that, is also keeping them in an insecure state.
compare to
https://www.cs.columbia.edu/~angelos/Papers/2007/SSARES_ACSA...
and
In that case, you have to run a service that is always running that is effectively keeping the e-mails available in the clear. Is it better security than keeping them in the clear on a central server? sure. But is it really any different than running my own smtp/imap/webmail server on aws without encrypting any emails which I access? In practice it would seem to be a similar threat model and I'm not convinced many would view that to be particularly secure, just a tad more private.
At least I'm pretty sure that's what I'm doing.
In the longer term / larger scale, it's not going to be sufficient though.