The first Assassin’s Creed game only had side missions because “the CEO’s kid played it”.
That’s the start of the wild story behind the addition of side missions to the 2007 game, as told by then Ubisoft fight system AI lead Charles Randall in a fascinating Twitter thread.
The first Assassin’s Creed contains a number of flag collecting missions, as well as Templar assassinations. These “additional memories” do not advance the plot.
“Assassin’s Creed 1”, All 100 King Richard flags locations in Kingdom Watch on YouTubeAssassin’s Creed – Additional Memories – Kill All Templars Watch on YouTube
It turns out Assassin’s Creed was ready to ship without these side missions when, just days before the game had to be sent to shops, “the CEO’s kid played the game and said it was boring and there was nothing to do in the game”.