In order to crimp connectors onto a wire, a special tool is required. Note: Pliers are not crimpers! Neither are hammers, vises, needle nose pliers or flat rocks. A good crimper when used correctly will make a cold weld between the wire and the barrel of the connector. If you were to cut a well executed crimp in half you would see a solid form of wire and connector. Using the wrong tool will not achieve a good crimp!
Why is this level of perfection required? A poor crimp leaves air pockets between the wire and connector. Air pockets allow moisture to collect, moisture causes corrosion, corrosion causes resistance, resistance causes heat, and may ultimately lead to breakage.
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