File with CRDOWNLOAD file extension is a type of temporary file. Downloaded files are saved to disk and stored in Downloads folder. Some people wrongly claim that downloaded files are not stored on the disk but in the operational memory or directly accessed by opening it on the server which hosts such file. Such files are saved in temporary files folder on user's disk.
Temporary folder also stores other types of files. Many applications use temporary folders to save some data for easier access or to speed up application processes. Temporary folders tend to grow in size very quickly if left unmanaged and thus the first step to speed up the computer is to clean temporary file folders. Removing data from temporary folders may slow down some applications, but it allows to regain large amount of disk space.