Was there any actual doctrinal or military research behind it? did having a stick give you advantage over a non stick style such as being able to throw it further?
Two main reasons:
The German stick grenade (let's use the Model 24 as an example, it's pretty much the de-facto stick grenade people think of) provides a greater effective thrown range. Effective range was roughly 60-100 feet, which is about a third greater than the spherical or cylindrical hand-held grenades (British Mills bomb, US Mk. II frag). You're right in assuming it gives an advantage for throwing, the stick provides a mechanical advantage. Most German grenade systems used a friction-style ignition (think dragging flint and steel together), and that cord and igniter needed to be protected, which is another benefit of the handle; you can't snag the cord and have an unwanted detonation if it's protected. The bottom cap comes off and the cord falls out, you give it a tug and throw. Slang for these among English speakers was "potato masher", because well, they sort of look like one.
The M24 and similar style grenades relied more on a concussive force than fragementation; as such the blast radius was smaller, and it didn't fling any significant amount of shrapnel. The "pineapple" grenades you see in cartoons and war movies are Mk. II fragmentation grenades and their analogues; they're meant to explode and whip a cloud of metal fragments everywhere.
Two different styles of grenade, two different philosophies of use. There are also rifle grenades, phosphorous grenades, the list goes on and on.
Couple of previous threads about this:
Quick summary: German grenades were made for offensive uses, they could be thrown farther, with better accuracy, and as you advanced.