Cheaper. Renting cloud infrastructure from an IaaS provider is almost always cheaper than relying on your own servers. Moreover, when you have no tests to run you won’t get charged for servers that are lying idle.
2Scalable. Cloud is nearly infinitely scalable. The only limitations on scaling your tests are likely to come from the underlying test automation software (and from the resource limitations of the VMs you are using).
3Reliable. Virtual servers always offer some level of reliability. That could be cold migration (where you power off, start a new instance and recover using the backup).
Or it could be hot migration of running servers. The better providers even offer forms of Disaster Recovery. This is in marked contrast to what happens if your own server fails.
4Realistic. This is one of the more subtle benefits of using cloud testing. Generally, you will be able to run your tests from multiple locations and, importantly, your tests are coming from an external network.
5Effective. With cloud-based testing, you get access to almost infinite reliable storage. This means you are able to store all your historical test results. In turn, this enables you to fully analyze any test failures or issues. It also allows you to observe trends in your testing and is useful when you are dealing with regressions.