First: Clones can perform various attacks all at the
same time. Indeed, imagine if you had three clones
in front of you all release different jutsus at you at
the exact moment. It's like breaking time frame.
Second: It is hard to tell the difference between the
real you and clones. Only the Sharingan is able to tell
such differences, or normally, people just godmode
this in battle.
Third: Being able to combo greatly with split-second
jutsus - imagine seeing several attacks coming at you
at all directions.
Fourth: Distraction in general - for example, a clone
used Thunder Up, and caused an explosion, the
enemy will be drawn to the noise and you could then
take advantage of this.
Fifth: Release of Genjutsu.
Sixth: Sacrifice for information.
Seventh: Increase of field of vision. Blind spots.
Eight: Human Shield.