AFAIK, all Shimano mechanical FDs for 2x11 drivetrains have 4 positions, for inner to outer: (4) L-trim, (3) Low, (2) T-trim, and (1) Top (using Shimano's terminology).

1. Top is for the big chain ring and the (physically) smallest 6 to 8 cogs of your cassette.
2. After shifting onto the next bigger cog of your cassette, your chain will get noisier, and releasing the FD onto the T-trim position will alleviate the noise. Where this happens on your cassette depends on the cassette range, the chain stay length, the chain line of your bike, and (to a tiny extent) the freehub and any shim thereon. Thus, T-trim is generally necessary at the big end of the cassette. If the stars align perfectly, T-trim may even let you cross-chain the big-big combo (on a road drivetrain) without additional noise. But the GRX chain line makes this a pipe dream.
3. Low is for the small chain ring.
4. Shimano dealer manual warns that you can expect chain rub when riding on the small chain ring and the 4 smallest cogs.
(a) If the chain is rubbing on the big chain ring, releasing the FD onto the L-trim position may alleviate this by increasing the clearance between the chain and the big chain ring, and net you the use of one additional cog.
(b) If the chain is rubbing on the FD cage, L-trim would likely make it worse, and is thus not useful.

So it is possible that you may not need L-trim while riding. However, I generally shift the FD to L-trim and the RD to the middle cog (of an 11 speed cassette) before I park it, to reduce the chain tension on the RD.