No, it just increases the chance. So... let's say you are in a place that has level 16, level 17, level 18 and level 19 pokemon.
Each has a 25% chance of appearing for this example under normal circumstances. Now... Put a Pokemon with pressure in the lead slot. Now the level 19 Pokemon will have a 37.5% chance of appearing.
Now... Let's say the Pressure Pokemon is level 19 and you activate a repel. Now, the chance of a high level Pokemon appearing is checked by the RNG and you have a much much higher chance of encountering the higher leveled Pokemon. It is most useful when a specific Pokemon you are looking for only comes at a specific, high level.