It makes a lot of sense to use iFlicks as the default application to handle video files on your Mac. This especially the case for video formats like avi or mkv that are not natively supported on macOS.
Note: a good alternative to handle these filetypes might be Watch Folder to automate conversion.
To set the default application to be used when opening a file, it is common to option-click on a file in Finder. After selecting Open With and Other... iFlicks can be selected and the option Alway Open With activated. After clicking on Open iFlicks will be set as the default application to open your video files.
On macOS Mojave these steps might lead to the message "filename" can't be opened because it is from an unidentified developer as seen below.
If this is the case you can choose option click on the video file and select Get Info. In the resulting window iFlicks can be set as the default application to handle your video file. Clicking on Change All will activate iFlicks as the default application for all video files of the same format.