Organelles benefit eukaryotic cells by creating compartments to separate different metabolic tasks.
One of the problems that cells face is having sufficient concentration of substrates for specific chemical reactions to occur. Having different reactions occurring in different organelles might benefit cells by creating locally high concentrations of specific substrates, along with needed enzymes and local conditions such as pH.
Another problem is one of cell size; having separate compartments may reduce the diffusion difference between the external environment and the organelle for specific metabolic pathways.
While evidence clearly supports the idea that mitochondria and chloroplasts arose from symbiotic intracellular bacteria, other organelles might have developed from an accelerated production of internal membranes along with embedded or enclosed enzymes that became available as mitochondria evolved to supply much larger amounts of available ATP.