Well we all need to remember about the times when the idea of trainable torpedo tubes was born.
The first trainable external TT (at least to my knowledge about European navies) were introduced by French in their 1922-design of REQUIN class submarines. During that times submarine warfare looked completely different than in for example 1941.
The idea behind was quite straightforward. At the time submarine before the sinking of enemy shipping was required to surface, stop the merchant vessel and put the crew into the life-boats. This was time consuming and submarine was vulnerable to any external threat when remaining still on surface. Thus trainable external TT were in theory suppose to resolve this issue since submarine could have been in constant movement and external TT would be aimed at the target.
Similar system was also developed in Dutch Navy (independently) and later in Polish Navy (as a consequence of building submarines in France and Netherlands).
French even got one step further and decided to use smaller (450 mm) torpedoes in external TT. Since those were supposed to be used only against the shipping it was better to use smaller (but cheaper and less heavy) and faster 450 mm instead of 550 mm torpedoes (though 450 mm was absolutely sufficient for the intended purpose). But this created logistics problem since sub would require 2 types of torpedoes so it wasn't developed further.
BTW similar question was already asked -
http://forum.axishistory.com/viewtopic.php?t=153302 
; a subject that piqued my interest ...........................
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