IANAL, but the GPL2 allows bundling and "tivoization" [0] with proprietary code, whereas the GPLv3 tries to prevent it. So, with v2, you can ship your proprietary application with an unmodified GPL'd tool along with it, but with v3 you can't. For v2, the copyleft applies to modifications of the tool, for v3 the fear is that the copyleft applies to all the software that touches the tool.

[0] https://en.wikipedia.org/wiki/Tivoization#GNU_GPLv3