Let’s say the build steps pass. Great - you’ve verified that the new code works! But then you discover that the code has an adverse effect on the performance of your application. Suddenly your site falls dramatically short of meeting your performance metrics.
If you’re running CI processes without running load tests, you’re only looking at one part of the equation - and you’re going to get an incomplete answer. If your app is going to be deployed to QA or production, it’s important to integrate load testing into your processes. Or, at the very least, take your tests one step beyond the functional testing on a daily basis so you can continually get a good indication of your apps’ performance.