The slit allows ambient gas to equalize the gas in the vial so that a vacuum is not created in the vial from the tight seal around the needle. If coring or clogging from a narrow needle or needle deflection from highly durable septa material is a concern, then choosing a pre-slit septum is a better choice.
A PTFE/Silicone with slitted septum allows for easier needle penetration as well as to release the vacuum that forms when a large volume of sample is withdrawn from a vial. This septum provides chromatographic characteristics similar to that of a septum without a slit except that the ability to withstand exposure to aggressive solvents is slightly lessened. Pre-slit septa are highly recommended improving injection to injection reproducibility with autosamplers withdrawing greater than 50µL of sample from a 2mL vial, due to possible cavitation (vacuum).