Once you've implemented all the energy efficiency work you can, you've changed your fleet, swapped out new boilers, put in high efficiency lighting and even reduced employee commuting and optimized manufacturing lines, how do you drive your carbon footprint even lower?
Why not install solar panels all over your facilities? Solar can be complex and expensive (>3yr simple payback is intolerable for many companies), and if you live in an area without a lot of sun, it might not make sense. With the pain of installing wind turbines, that might not work either.
Your last shot might be through purchasing RECs or Carbon Offsets. After all, doesn't it make sense to incentivize another project to move forward through meeting its hurdle rates--something that can be done by subsidizing these projects. (take a look at 3Degrees for a great summary) Think RECs don't work? Look at the explosion of solar power in MA lately since SRECs are at $529/MWh. Another option is high-quality offsets from a project that wouldn't move forward without this extra support. Thus the phrase, "additionality".
The trouble I've seen in practice is that employees and company executives don't buy into the additionality of the project, thus rendering the money in the same category as snake oil and used cars. How do we know exactly what we're getting? What is the business benefit? Why throw money at an external project when we have internal capital needs. Answers to these questions are tough to pin down.
On the positive side, these decisions and the discussions around them at least raise awareness about the intricacies of renewable energy projects and practicalities of carbon footprint improvement.