HPLC solvents that are exposed to the atmosphere, even for a short time have the ability to absorb compressible carbon dioxide CO2 and oxygen O2.

When your HPLC is operating with low back pressure, the actual linear flow of materials could be different then when you have higher back pressure. This is due to the compressibility of air versus the compressibility of gas. This means that your HPLC results could different retention times and different peak shapes.

Also, noisy baselines occur which can lead to very unreliable results in both quantitation and retention times. Some columns, most notably polymeric columns, do not tolerate outgassing very well. It is therefore always recommended to keep your solvents optimally performing by degassing them and keeping them degassed.