Hi my friend, you should always remove redundant on your device for better computer performance because too much data or junk file accumulation makes your computer lag or run slowly. You're recommended to move the big yet seldom-used files to an external hard drive and then delete the messy and useless files for more storage space.
Media files runs too much space so it's a good choice to move them to the hard drive. Audio like music, pictures like HEIC files, videos like MST files and FLV files can be moved into a new folder on your external hard drive.
Programs can't be move without reinstallation. With the standard Windows installers, you may need to copy the program folders, uninstall the program, and then reinstall it on the external hard drive. Note that all programs installed on the external hard drive can't run unless you connect it to a computer.
As to useless files, you can perform a deletion work with several clicks. Just select the files you want to delete and then right-click them to move to Recycle Bin or Mac Trash.
Step 1: Select a volume from the startup disk usually labelled as macOS - Data, macOS, or Macintosh HD, or Macintosh HD - Data.
Step 2: Click "Clean Junk" from the toolbar.
Step 3: Select a folder from the left panel and select files/folders you want to delete.
Step 4: Click "Clean" and then click "Ok". You will fail to clean certain files if you don't have permission to do so.