It is not mandatory. If you upgrade to Catalina it will reformat to APFS automatically. If you want your internal drives to be APFS you can convert those by restarting your Mac by using Command + R to boot into recovery mode. Then, from Disk Utility>View>Show All Devices> Select your parent drive (or external) then click Edit and Convert to APFS. It will not impact your files. Before doing any conversion be sure you have a backup just in case.