It's the same as the difference between locking a target with the TGP, and making the TGP the SPI. If you release a guided bomb it won't hit the target your TGP is tracking, it'll hit your SPI. If you follow steering cues to your SPI, you won't find yourself at the target location, etc.
If you hook an object on the TAD, then you'll get some cues on your HUD to help you find it, and the ability to create a waypoint from it, but your SPI will remain whatever it was previously (TGP, STPT, or whatever). If you make the TAD your SPI generator, then whatever you have hooked will be your SPI.
Also, you can't make the TAD your SPI without hooking something, since having an object hooked is the only way for it to generate a position.