Eukaryotes are classified into unicellular and multicellular,what makes this classification so important?
Well,My book says this basis of classification marks a fundamental difference in body design because of specialisation of cell types and tissues.[USfundamentaYgggdrasil[/USER] is talking about are differences in the way taxonomists cubbyhole organisms.
His point is that using evolutionary (genetic) information to classify liging things is more useful and accurate - this is usually called phylogenetics. The idea that chimpanzees and modern humans share a large percent of their DNA -> an example.
Another way to classify is morphological. Lump everything that looks a lot alike into one pile. This approach used: structure of a flower, life cycles, skeletal structure, dentition in mammals. This has issues compared to the phylogenetic approach. What is the closest living plant to the lotus? Using the look alike system almost any scientist in 1980 would and did say - water lillies. DNA says something different. Plane tree is the answer. They DO NOT look much alike.
Okay. So what has happened to classification? It has changed drastically in the past 25 years, think: the lotus thing.
Now we are down to unicellular versus multicellular. What do you suppose this tells us? A whole lot less than a taxonomist would have said 25 years ago. Some of these groups are considered more closely related than we thought before. Others that were previously lumped together have now become widely separated. The yeast-mushroom example (above post) is a good one in this context, too.
Direct answer to your question - you may be reading someone who follows an older approach. Or - for teaching beginner non-technical classes sometimes teachers do things like make 'things easy', rather than any other approach.