You don't use trail braking to slow down. You use it to control the posture of the chassis through turns, which effects both mechanical grip and steering.
On bikes such as tall, long-travel adventure bikes that are prone to pitching and weight transfer, trailing the rear helps keep the chassis level. If I want to scrub off speed before a turn, I'll do it with both brakes before entering the turn, but trail the rear very lightly to the point past the apex where I hit the power again.
Trail braking isn't about slowing down. You should have done that before you commit to the turn - ie. slow in, fast out.
It's less valid these days when so many bikes have semi-active suspension which does the chassis leveling for you (up to a point), but it's very useful on older generation bikes like the 1190 which can turn into a see-saw under aggressive braking.