The only difference is that -Syyu forces the database to update
To explain what database means in short, it tells pacman what packages are available in different repos (e.g. core, extra). In some rare cases, the time of the database update may be incorrectly marked, and pacman would not know there are new packages/versions. -Syyu should be used in this case.
Do you actually need - Syyu or is - Syu fine? I have only really used the latter.
The only difference is that -Syyu forces the database to update
To explain what database means in short, it tells pacman what packages are available in different repos (e.g. core, extra). In some rare cases, the time of the database update may be incorrectly marked, and pacman would not know there are new packages/versions. -Syyu should be used in this case.