Hi,
I guess because this is generic approach that was designed rather for handling trust between separate boxes than when running both modules on one box. I can't recall it right now (was doing lab config some time ago) but LTM is storing certificates for device authentication in System ›› Device Certificates : Device Certificate ›› Device Certificate and GTM in DNS ›› GSLB : Servers : Trusted Server Certificates. This second location is not by default populated with certificate from the first (I am pretty sure but not 100%) so bigip_add is used to transfer it even if this is one device. Anyway I did this certificate exchange manually by exporting and importing certificates - it's another way to do that without bigip_add - but probably more cumbersome :-)
That is more or less the reason as far as I understand it.
Piotr