Most essays at a college level require a bibliography or reference page as well as in-text citation of some sort. It is important to gather the needed information as you work. Sometimes searches and resources are hard to find again, and it saves time at the end of the process when deadlines are looming.
There is also the added benefit of building on searches you have done to do further research. A thorough record can save time and effort in that process.
Because so many sources are formatted electronically, it often easier to keep the records of research electronically as well.