The testing conditions that are used when developing websites aren't the same as what users experience - especially when they are on a mobile device which has lower bandwidth, power, etc. It is important to ensure that the site is usable from the user's perspective. This kind of testing is usually required for you to see how your site performs when there is higher latency, packet loss, single points of failure (when a domain is unreachable) and so on..

Though these are great tools to help us test, I still strongly recommend using real devices with real network conditions to do tests like these.