Video without RAM contention, for one: https://retrocomputing.stackexchange.com/q/22257/278

> The 6502 needs access to memory only half the time, during the what is usually called the ϕ2 phase of the clock. The other half of the time, so long as you tri-state the CPU address pins,² the bus and memory are available to other systems.

> The Z80 does not have such a well-defined, synchronous system of accessing RAM. Many cycles don't need RAM access, but many do, and which particular ones do and do not depend on the instruction mix.

> There is no "do nothing" solution to this constraint, and Z80-based systems that wanted consistent video output would have to solve this one way (hardware) or the other (software).