In Spring 18 Salesforce released the ability to clone a Sandbox. It has been asked from both Admins and Developers for some time but now you are able to take advantage of exciting new feature! In fact, this request was 7 years in the making and have over 11k votes on the Ideas page.
Sandbox cloning simplifies having multiple concurrent streams of work in your application life cycle. This will allow you to build work in one sandbox then replicate those changes so that another system admin or developer can start from a developed point in time without losing the changes that the first admin or developer made. It will also allow you to go from Development to QA, Staging then Production making sure you test all the touch points and delivering as bug free environment as possible. Contingent on your Software Development Life-cycle, you can have changes sitting in the sandbox from a Sprint or two ago.
One notable factor to consider is that today a full copy sandbox to a developer sandbox is still not possible.
This is personally my best feature in Spring 18 and you should investigate if it can be used in your development strategies moving forward. Good job Salesforce for continuing to deliver features in the platform in which customers are truly interested in using.