Screw bases have a number of advantages compared to the bayonet fit type:
- Screw bases are more suitable for small size bulbs
- A bulb fully screwed home is more secure than a bayonet fit bulb
- Moisture and debris is less likely to contaminate the contacts of a screw base bulb
- Sprung contacts are not necessary on screw base bulbs (the spring tension must not only ensure a good contact but it must also hold the bulb securely in the bayonet so there is a compromise)
Mainly used for flashlights, bicycle lights etc.