Rear lights for mountain biking are less important as you don't need to be visible to traffic while on the trail. However, if you do need to ride on the road, whether as a connection between trails or in case of emergency, you will need a rear light so that cars can see you in the dark. Rear lights are also a nice safety feature for riding on the road in the daytime. A rear light will make you more visible to drivers.
A rear light doesn't need to be as bright as your front and we recommend looking for a 70-lumen rear light for general countryside and city riding. All rear lights will have at least a constant and flashing mode these days although different brands will offer various power modes and flashing patterns that are designed to increase visibility. Daylight modes differ from standard night modes as they rely on disruptive flash patterns to attract attention.
Fitting commonly uses silicon straps to mount lights to a seatpost, and is easy to attach or remove quickly. While most lights will accommodate a wide range of seat post sizes it is still worth considering fitting options. Especially important if you have a dropper seatpost or are looking to mount your light to seat stays, helmet, or saddlebag.