Once upon a time, two globs of primordial ooze met and fell in like. They then decided to form a Dark Army to compete with other globs of primordial ooze! Dark Armies of primordial ooze have evolved at least twice, once in plants and once in animals. What about multicellularity is so great?
Most modern life forms have cells contained by phospholipid barriers. There is probably a maximum size limit to the amount of space that can be contained inside the phospholipid barrier. Until the evolution of multicellular organisms, that meant that the size of the organism was limited by structural components of the cell. After this evolutionary leap forward, the size of the organism was no longer limited in this fashion. Multicellular organisms would therefore be able to out-compete other organisms where size confers an evolutionary advantage.
All life would begin as single celled organisms, but on most planets, I think this advent of multicellularity would occur. Your assignment, O (hypothetical) reader, is to comment on environmental conditions where large organism size would or wouldn’t impart an evolutionary advantage. Fred Meyer has a 1 hour internet time limit, so tissue differentiation will have to wait for next time!