Travis uses virtualenvs as toplevel containers for running CI.  Some projects need (or would like to) set up venvs as part of their test suites.  On example is https://github.com/airspeed-velocity/asv, which profiles a project's speed across revisions; in order to do so it sets up an environment for each revision to run the test suite.  Currently, it can either set up a virtualenv or a conda env, but it would make sense for it to be able to set up a venv too (https://github.com/airspeed-velocity/asv/pull/526).  But the unability to nest a venv into a virtualenv makes this impossible to test on Travis.