Why does kmail insist on having ldap available?

I've just noticed my Gentoo system updating openldap and looked into what 
needed it. All dependencies were conditional except for kldap, which requires 
it. Then I found that six KDE apps insist on kdap and thus openldap.

My question: why is ldap not optional? What proportion of user installations 
actually use it?

-- 
Gentoo testing system
gcc 9.1.0, sys-kernel/gentoo-sources 5.2.2
QT 5.12.4, KDE frameworks 5.60.0, KDE plasma 5.16.3
KDE apps 19.04.3 incl KMail 19.04.3 (5.11.3), akonadi 19.04.3
dev-db/mariadb-10.3.16
x11-drivers/xf86-video-amdgpu 19.0.0
dev-libs/amdgpu-pro-opencl 18.20.684755