Among many other reasons: because the web interface is utterly incapable of sending and receiving unmangled patches, or more generally sending and receiving plain text without wrapping.
Also: because people might want security, such as GPG.
I don't think the parent post was arguing about that, I think he was arguing that if these are important features to you, why choose gmail in the first place?
Upsides: it's ubiquitous, well-provisioned, has very high uptimes (a few outages, those tend to become national or international news), and, for now, good support of IMPAPS or POPS access for standalone email clients (which I use on my phone and desktop).
Downsides: Snooping, NSA honeypot, Chinese government hacker honeypot, etc., etc.
Moreover, if you regularly use GPG, the GMail web interface would be nearly unusable, no? So it seems you'd be much better off with nearly any other email provider.
Most GPG messages I get are signed, not encrypted, and I usually don't need to check the signature. So the web client is fine, but it's still good to have desktop client access.
Also: because people might want security, such as GPG.