1. All-in-one device.
There are many devices that a reptile keeper can buy in a ZOO shop. Temperature regulators, thermometers, humidity meters, timers, dimmers, etc. Every device takes space and costs money, and stuff for hobbyists are expensive, I mean really expensive. Regular halogen bulb is $1 and a halogen 'for reptiles' is $10. Buying 10s of devices makes you to reserve extra shelf for all of them and you have to manage all cables. There are 'professional computers' for reptile tanks, but spending $500 for a single device is not always within a budget of a hobbyist.
2. One device to support all reptile environments - day/nigh, dry/humid.
One device has to be able to provide the best environmental conditions for all reptiles (snakes, lizards). Some of them are active during day, some of them during night. Some live in the desert, some in the jungle. The device needs to control temperature, humidity and light.
You could manually switch temperature setting twice a day. That is perfectly fine - you are the keeper and this is your responsibility. But everyone deserves a vacation or sometimes you just cannot come back home in time. So, the device has to be able to control the environment autonomously according to your settings.
4. Easy to use and safe.
When you need to check something (temperature value, settings), you need to check it fast and change it easily. I know that nowadays you can control anything from the app. But I am taking info account accessibility. There are lots of reptile keepers that are in their silver years, so 'manual' UI is a must.
5. Providing the best natural conditions for reptiles.
There are many on/off temperature regulators based on relays. But this is not natural to just blink 0/1 with a bulb. Also, in nature temperature changes between day and night. That is why the device has to mimic environment as in nature.