One of the advantages of fixed hardware is you can tune for that hardware directly. We used to bake content and design code with specific cache line sizes in mind. If you know you have a fixed memory budget for graphics/sound/etc you can make specific tradeoffs on where you spend that budget. PCs until recently did not have UMA and so you were bound by bus speeds(although this applied to some consoles too but you could exploit it if you knew how, ex: stream audio data from graphics memory and long as you stayed under the total bus bandwidth you got extra memory for the system if you needed it).