Why did Tobirama create Edo Tensei?
That is correct. Edo Tensei was designed to be used as a weapon. Edo Tensei has several uses:


Using your Enemies' allies against them has a deep impact on morale (Imagine having to fight to the death against your best friend/brother/father)
Regenerating soldiers can use suicidal moves repeatedly (Although it does seem like some techniques leave a permanent damage, even on an Edo Tensei'd body, like Itachi's Izanami).
Increasing your numbers is always a good thing.


Tobirama created this technique to battle in the wars between clans/villages during his time alive.