Revenue is a metric that most businesses care deeply about so, as Michael says, it’s typically a good candidate for a primary goal. However, does not have to be a primary goal. I thought I'd chime in to link you to a couple of Knowledge Base articles that might provide a little more context.
Optimizely recommends that you choose a primary goal that aligns closely with your business goals; it should be something you really care about. This is because Stats Engine calculates the primary goal separately from your other goals and makes sure that it always has the highest statistical power. You can read more about why in this article.
However, there are many other goals that you can set as a primary goal: clicks on a call-to-action button, form submissions, cart-clicks, and more. It doesn’t have to be revenue, and nothing is inherently thrown off if you don’t use revenue. It all depends on how you defined your hypothesis and what conversions matter the most. If you need more information on how to decide on a primary goal, look here.
For example: our e-commerce director is keen on revenue. I am more geared towards UX so I look at engagement and clicks.
Optimizely needs a primary goal to make it's calculations - but we use many at once.