Possibly high sample injection volume and/or high sample concentration can be "pre-focussed" on guard column/"pre-separated", which will lead to sharper/more focussed peaks.
Possibly organic content of sample solvent is high
In theory, assuming one uses the same phase chemistry for guard and column, efficiency should simply improved by the ratio of guard/column lengths, i.e 5-20%. Yet in practice, even with identical phase chemistries it is not impossible that the guard and column have slightly different selectivities which might sometimes cause improved separation in some particular case.
off course it help to improve the separation process in HPLC .It's look like when some one want to smoke cigarette he use a double filter to removed unwanted comounds. so the guard column used to protect the main column in order to improve the separation process of the sample.
Since a guard column is nothing more than a miniature HPLC column, it effectively extends the length of the main column. And If the column chemistry is not exactly the same, it can add a little more separation on a chemical/selectivity basis too.
1) you pretty much concentrate your sample in the pre column
2) most of the unwanted compounds like salts are flushed away, thereby protecting your analytical column and also increases the efficiency of the column
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