Why do we need the "nop" I.e. No operation instruction in microprocessor 8085?
One use of NOP (or NOOP, no-operation) instruction in CPUs and MCUs is to insert a little, predictable, delay in your code. Although NOPs don't perform any operation, it takes some time to process them (the CPU has to fetch and decode the opcode, so it needs some little time do do that). As little as 1 CPU cycle is "wasted" to execute a NOP instruction (the exact number can be inferred from the CPU/MCU datasheet, usually), therefore putting N NOPs in sequence is an easy way to insert a predictable delay:

\$ t_{delay} = N \cdot T_{clock} \cdot K\$

where K is the number of cycles (most often 1) needed for the processing of a NOP instruction, and \$T_{clock}\$ is the clock period.

Why would you do that? It may be useful to force the CPU to wait a little for external (maybe slower) devices to complete their work and report data to the CPU, i.e. NOP is useful for synchronization purposes.

See also the related Wikipedia page on NOP.

Another use is to align code at certain addresses in memory and other "assembly tricks", as explained also in this thread on Programmers.SE and in this other thread on StackOverflow.

Another interesting article on the subject.

This link to a Google book page especially refers to 8085 CPU. Excerpt:


  Each NOP instruction uses four clocks for fetching, decoding and executing.


EDIT
(to address a concern expressed in a comment)

If you are worrying about speed, keep in mind that (time) efficiency is only one parameter to consider. It all depends on the application: if you want to compute the 10-billionth figure of \$\pi\$, then perhaps your only concern could be speed. On the other hand, if you want to log data from temperature sensors connected to a MCU through an ADC, speed is not usually so important, but waiting the right amount of time to allow the ADC to correctly complete each reading is essential. In this case if the MCU doesn't wait enough it risks to get completely unreliable data (I concede it would get that data faster, though :o).