There are many threads on this topic. Search Darkroom Automation F-stop timer and RH Designs Stopclock timer.

I have the latter and consider it indispensable. Someone gave a link to its manual which can be a bit challenging until you have a timer in hand and start using it. I have the same feeling for the DA timer's manual which I have read also.

With a regular timer, you typically use 3s or 5s test strips which are fine except that the zone difference between each strip decreases the exposure time increases. With an f-stop timer, the zone difference between each stop is the same. I set mine for 1/6 stop difference between each step. The time has a test strip mode so I keep adding as many 1/6 stop strips as I want. Now I tend to see the print tones in stops and that make dodging and burning more intuitive.

For burning, the timer has numerous memories so you an easily program many steps for burns. It is easy for me to make a print with half a dozen burns of various times without having to remember seconds. On my printing notes, I include the base exposure and each burn is in stops, e.g. +1/6, +11/6. For the +11/6 burn, I just push the program button to enter "Program 1" and then punch the up button 11 times.

The timer has two channels so it works great for split grade printing. It has a function to tie the two channels together so that if you adjust the base exposure in one channel, it adjusts the exposures on the other channel as well to maintain the same contrast grade (I don't use that one).

It has a drydown mode so that you can program in a drydown percentage when you are ready to expose final prints. You click the "Compensate" button and it automatically reduces the programmed exposure times by the drydown percentage.

I believe it has a metronome function built in but I don't use that either. While I appreciate the simplicity of a metronome, I don't have the attention to count to 38, then 7, then 12, then 22, etc.

Some of the timers are compatible with exposure and contrast meters.

Mine has a probe that can be fitted to a cold light to adjust the timer to any variations in light output.

Even if you like a very low tech darkroom, you will appreciate an f-stop timer because its technology truly simplifies the process once you get the hang of it.