In Guns, Germs and Steel, what did what did steel do that was important?

This is a good question, because much of what steel does
seems to be the same as what the guns do.  But Diamond is making somewhat of a
distinction between the two.

To Diamond, the "steel" part
of the title refers to technology that is not necessarily military technology.  Here and
there in the text he refers to the importance of steel tools.  These tools made it
possible for the dominant cultures to create things such as modern ships on which they
could cross the seas to conquer the societies that did not have the same level of
technology.

So, while military weapons are a form of
technology, they are not the only form.  Diamond is using the term "steel" to refer to
other kinds of technology like plows and other sorts of tools that help the dominant
societies gain power.