They are somewhat not. But..
Let's say you're making a beer you've done lots of time before, then you usually just want to take an OG reading just to confirm that you landed your OG.
But, If I'm making a beer with a new grist, I like to take a gravity reading just to see if my calculations of OG were at least somewhere in the ballpark. As you do or do not know, more malt gives you lower efficiencies, so grists with different amount of malt will give you a different efficiency. So taking a gravity reading preboil will give you an answer to if you'd want to adjust with either DME, LME, or a tweaked boil-time, or with a water-dillution, to hit your targeted OG.
Using BIAB my mash thickness is typically in the 2.5qt/lb range or higher, that may be why my efficiencies are easier to predict?