It's generally assumed that during the evolution of lifeforms, there must have been some stage when single cells (unicelluar organisms) grouped together to form multicellular life.
There are at least nine different major theories regarding how (and why) this might have happened. See: Wikipedia
The subject is highly controversial - not least because, in terms of numbers of species, the vast majority of current lifeforms are unicellular.
Multicellularity allows an organism to exceed the size limits normally imposed by diffusion: single cells with increased size have a decreased surface-to-volume ratio and have difficulty absorbing sufficient nutrients and transporting them throughout the cell. Multicellular organisms thus have the competitive advantages of an increase in size without its limitations. They can have longer lifespans as they can continue living when individual cells die. Multicellularity also permits increasing complexity by allowing differentiation of cell types within one organism.
Whether these can be seen as advantages however is debatable.
(Source Wikipedia, as above)
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