Later material, not previously tendered,
can become relevant when it is used to rebut evidence given by a witness,
which is later shown, in cross, to be, at least, not wholly true, and at worst, an out and out lie.
In short and simple...
If you say something in evidence (eg: "I was not there"), then
the other side can put on evidence to show the opposite.
They are not always and automatically required to disclose that in advance.