These silicone heater are specially designed for this application offering superior energy efficiency over standard formed rod heating elements. They are of a high watt density design and are fitted with temperature limiters which are designed to protect the heaters from over heating should the water level evaporate below the safe working minimum.
Normal formed heating elements are typically twice the power rating of its silicone heater counterpart and as the element surface contact is limited will reduce the efficiency even further. We would recommend the use of these heating elements as best practice as they are better for the environment and reduces the users operating costs.
The use of float level switches are recommended to prevent water levels falling below suitable levels, see link here.