Griffin wrench great for low clearance - Griffin Armament Griffin Armorers Wrench

Wrench flats are great to have, but they are not always accessible for a few reasons.
If you have to install your muzzle device AFTER you install your rail, and there is not much gap between your rail of choice and the device, then getting a wrench on to it can be impossible or problematic.
Also, if you need to time your device, such as with brakes, very often you will need accu-washers, shims, or similar spacers. These often butt against your wrench flats and interfere with properly gripping your muzzle device.

This Griffin wrench has been great. It is very thin but strong and easily clears rails, handguards, and washers, allowing you to properly grip your muzzle device when access to the wrench flats is restricted, and allowing you to get a proper torque on to your device. Considering the low cost, there is little to no reason not to have one in your kit.