There is no such thing as "sealing in the juices" - that's just a really delicious sounding cooking myth.

The best thing you can do to keep juices in your meat is to not overcook them and disturb it as minimally as possible (and for God's sake don't squish it with a spatula!). So place it down, flip it once, and remove as soon as it hits proper temp - that's the key to retaining juices.

Searing meat adds flavors and textures that are certainly pleasant but it has nothing to do with juices.

Personally I prefer my burgers on a BBQ rather than a flat top as I find flat top burgers taste too "overly charred" and one dimensional for my liking while BBQ burgers give you a nice amount of char while still hitting you with primarily the flavor of the meat itself. I find you lose a lot of the burger flavor when the primary flavor is a crispy black sear on all sides. It's a delicious flavor, but I prefer the flavor profile of a burger cooked over a thick steel grate. To each their own :)