If you write
tell application "Finder"
tell application "System Events"
end tell
end tell
you need to keep the "Finder" tell structure open for as long as you need the "System Events" tell structure. This might mess up your code and it could cause problems if multiple apps have conflicting commands.
"System Events" rarely or never conflicts with anything and is needed relatively often. It makes sense to have a very long "System Events" tell structure but it doesn't make sense to have a very long "Finder" tell structure.
Whether you use process or application has nothing to do with it. Both can access GUI scripting, but you may need System Events to do this while controlling an app other than Finder.