The implosion design is tricky. You need to arrange and detonate the explosives precisely to compress the core evenly from all sides, otherwise it shoots out the side or otherwise doesn't go bang the way you want it to. Hence the test.

That trickiness can be a good thing. Almost all modern weapons use the implosion design, partly because it's much safer. With a gun-type design, an accident could easily cause the two pieces to contact each other, resulting in an unwanted detonation. With an implosion design, accidentally setting off the explosives is very unlikely to set them off with the correct timing, so you'll probably just lose the core.

The implosion design is also a lot more efficient. Little Boy used 64kg of uranium. Fat Man used just 6.2kg of plutonium and even got a bigger bang out of it.