What does the pressure in the HPLC(High performance liquid chromatography) do?
The pressure in HPLC systems is not an intended feature, but a consequence of how the system works.

There are several adjustable parameters/properties of a HPLC/UPLC system that will influence on the internal pressure inside the chromatographic column. Some of those are:


Solvent flow:  A greater flow of solvents mean a faster chromatophy. On the other hand, faster runs also mean lower efficiency. An increase in flow leads to an increase in pressure.
Column length: A bigger column will improve the separation efficiency, but also increase the duration of the run. Bigger columns leads to a greater pressure.
Particle diameter: The lower the particle, the greater the separation efficiency, but this also leads to a greater pressure.
Particle pore size: This has the same effect as above, but is not the same thing.


Optmizing this parameters and/or improving the technology related to them is what generally drives the research in the HPLC/UPLC development field, besides of course find the appropriate type of column, solvent gradient, etc. You can't simply make a system where the pressure is as high as possible, or some connectiong will eventually burst. On the other hand, it's not interesting to have a chromatographic run that lasts several hours, or doesn't really separate the molecules, specially when dealing with hundreds or thousands of different analytes.

There is much more to the above in several books and thousands of papers, including very detailed descriptions, experiments and calculations that show a much more precise relation between those and other parameters.